diff --git a/.build/build.sh b/.build/build.sh
new file mode 100755
index 0000000..b926661
--- /dev/null
+++ b/.build/build.sh
@@ -0,0 +1,309 @@
+#!/bin/bash
+# PicoDeprecated -- build.sh
+# Builds a new PicoDeprecated release and creates release archives.
+#
+# This file is part of Pico, a stupidly simple, blazing fast, flat file CMS.
+# Visit us at https://picocms.org/ for more info.
+#
+# Copyright (c) 2019  Daniel Rudolf <https://www.daniel-rudolf.de>
+#
+# This work is licensed under the terms of the MIT license.
+# For a copy, see LICENSE file or <https://opensource.org/licenses/MIT>.
+#
+# SPDX-License-Identifier: MIT
+# License-Filename: LICENSE
+
+set -eu -o pipefail
+export LC_ALL=C
+
+# env variables
+PHP="${PHP:-php}"
+export -n PHP
+
+COMPOSER="${COMPOSER:-composer}"
+export -n COMPOSER
+
+if ! which "$PHP" > /dev/null; then
+    echo "Missing script dependency: php" >&2
+    exit 1
+elif ! which "$COMPOSER" > /dev/null; then
+    echo "Missing script dependency: composer" >&2
+    exit 1
+elif ! which "git" > /dev/null; then
+    echo "Missing script dependency: git" >&2
+    exit 1
+elif ! which "gh" > /dev/null; then
+    echo "Missing script dependency: gh" >&2
+    exit 1
+elif ! which "rsync" > /dev/null; then
+    echo "Missing script dependency: rsync" >&2
+    exit 1
+elif ! which "jq" > /dev/null; then
+    echo "Missing script dependency: jq" >&2
+    exit 1
+fi
+
+# parameters
+BUILD_NAME="pico-deprecated"
+BUILD_PROJECT="picocms/pico-deprecated"
+BUILD_VERSION=
+
+PICO_PROJECT="picocms/pico"
+
+PHP_VERSION="7.2"
+
+# options
+VERSION=
+PUBLISH=
+NOCHECK=
+NOCLEAN=
+
+while [ $# -gt 0 ]; do
+    if [ "$1" == "--help" ]; then
+        echo "Usage:"
+        echo "  build.sh [OPTIONS]... [VERSION]"
+        echo
+        echo "Builds a new PicoDeprecated release and creates release archives."
+        echo
+        echo "Help options:"
+        echo "  --help      display this help and exit"
+        echo
+        echo "Application options:"
+        echo "  --publish   create GitHub release and upload artifacts"
+        echo "  --no-check  skip version checks for dev builds"
+        echo "  --no-clean  don't remove build dir on exit"
+        echo
+        echo "You must run this script from within PicoDeprecated's source directory."
+        echo "Please note that this script will perform a large number of strict sanity"
+        echo "checks before building a new non-development version of PicoDeprecated."
+        echo "VERSION must start with 'v'."
+        exit 0
+    elif [ "$1" == "--publish" ]; then
+        PUBLISH="y"
+        shift
+    elif [ "$1" == "--no-check" ]; then
+        NOCHECK="y"
+        shift
+    elif [ "$1" == "--no-clean" ]; then
+        NOCLEAN="y"
+        shift
+    elif [ -z "$VERSION" ] && [ "${1:0:1}" == "v" ]; then
+        VERSION="$1"
+        shift
+    else
+        echo "Unknown option: $1" >&2
+        exit 1
+    fi
+done
+
+# check options and current working dir
+if [ ! -f "./composer.json" ] || [ ! -f "./PicoDeprecated.php" ]; then
+    echo "You must run this from within PicoDeprecated's source directory" >&2
+    exit 1
+elif [ "$(git rev-parse --is-inside-work-tree 2> /dev/null)" != "true" ]; then
+    echo "You must run this from within a non-bare Git repository" >&2
+    exit 1
+fi
+
+# parse version
+function parse_version {
+    VERSION_FULL="${1#v}"
+
+    if ! [[ "$VERSION_FULL" =~ ^([0-9]+)\.([0-9]+)\.([0-9]+)(-([0-9A-Za-z\.\-]+))?(\+([0-9A-Za-z\.\-]+))?$ ]]; then
+        return 1
+    fi
+
+    VERSION_MAJOR="${BASH_REMATCH[1]}"
+    VERSION_MINOR="${BASH_REMATCH[2]}"
+    VERSION_PATCH="${BASH_REMATCH[3]}"
+    VERSION_SUFFIX="${BASH_REMATCH[5]}"
+
+    VERSION_STABILITY="stable"
+    if [[ "$VERSION_SUFFIX" =~ ^(dev|a|alpha|b|beta|rc)?([.-]?[0-9]+)?([.-](dev))?$ ]]; then
+        if [ "${BASH_REMATCH[1]}" == "dev" ] || [ "${BASH_REMATCH[4]}" == "dev" ]; then
+            VERSION_STABILITY="dev"
+        elif [ "${BASH_REMATCH[1]}" == "a" ] || [ "${BASH_REMATCH[1]}" == "alpha" ]; then
+            VERSION_STABILITY="alpha"
+        elif [ "${BASH_REMATCH[1]}" == "b" ] || [ "${BASH_REMATCH[1]}" == "beta" ]; then
+            VERSION_STABILITY="beta"
+        elif [ "${BASH_REMATCH[1]}" == "rc" ]; then
+            VERSION_STABILITY="rc"
+        fi
+    fi
+}
+
+export COMPOSER_ROOT_VERSION=
+
+BUILD_VERSION="v$("$PHP" -r 'class AbstractPicoPlugin {} require("./PicoDeprecated.php"); echo PicoDeprecated::VERSION;')"
+
+if ! parse_version "$BUILD_VERSION"; then
+     echo "Unable to build PicoDeprecated: Invalid PicoDeprecated version '$BUILD_VERSION'" >&2
+     exit 1
+fi
+
+if [ -z "$VERSION" ]; then
+    GIT_LOCAL_HEAD="$(git rev-parse HEAD)"
+    GIT_LOCAL_BRANCH="$(git rev-parse --abbrev-ref HEAD)"
+    DATETIME="$(date -u +'%Y%m%dT%H%M%SZ')"
+
+    VERSION="v$VERSION_MAJOR.$VERSION_MINOR.$VERSION_PATCH"
+    [ -z "$VERSION_SUFFIX" ] || VERSION+="-$VERSION_SUFFIX"
+    [ "$VERSION_STABILITY" == "dev" ] || VERSION+="-dev"
+    VERSION+="+git.$GIT_LOCAL_HEAD.$DATETIME"
+
+    if ! parse_version "$VERSION"; then
+         echo "Unable to build PicoDeprecated: Invalid build version '$VERSION'" >&2
+         exit 1
+    fi
+
+    COMPOSER_ROOT_VERSION="dev-$GIT_LOCAL_BRANCH"
+else
+    if ! parse_version "$VERSION"; then
+         echo "Unable to build PicoDeprecated: Invalid build version '$VERSION'" >&2
+         exit 1
+    fi
+
+    if [ "$VERSION_STABILITY" == "dev" ]; then
+         COMPOSER_ROOT_VERSION="$(jq -r --arg ALIAS "$VERSION_MAJOR.$VERSION_MINOR.x-dev" \
+            '.extra."branch-alias"|to_entries|map(select(.value==$ALIAS).key)[0]//empty' \
+            "composer.json")"
+    fi
+
+    if [ -z "$COMPOSER_ROOT_VERSION" ]; then
+        COMPOSER_ROOT_VERSION="$VERSION_MAJOR.$VERSION_MINOR.$VERSION_PATCH"
+        [ -z "$VERSION_SUFFIX" ] || COMPOSER_ROOT_VERSION+="-$VERSION_SUFFIX"
+        [ "$VERSION_STABILITY" == "stable" ] || COMPOSER_ROOT_VERSION+="@$VERSION_STABILITY"
+    fi
+fi
+
+# build checks
+if [ "$VERSION_STABILITY" != "dev" ]; then
+    GIT_LOCAL_CLEAN="$(git status --porcelain)"
+    GIT_LOCAL_HEAD="$(git rev-parse HEAD)"
+    GIT_LOCAL_TAG="$(git rev-parse --verify "refs/tags/$VERSION" 2> /dev/null || true)"
+    GIT_REMOTE="$(git 'for-each-ref' --format='%(upstream:remotename)' "$(git symbolic-ref -q HEAD)")"
+    GIT_REMOTE_TAG="$(git ls-remote "${GIT_REMOTE:-origin}" "refs/tags/$VERSION" 2> /dev/null | cut -f 1 || true)"
+    PHP_VERSION_LOCAL="$("$PHP" -r 'echo PHP_MAJOR_VERSION . "." . PHP_MINOR_VERSION;')"
+
+    if [ "$VERSION" != "$BUILD_VERSION" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but PicoDeprecated indicates $BUILD_VERSION" >&2
+        exit 1
+    elif [ -n "$GIT_LOCAL_CLEAN" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but the working tree is not clean" >&2
+        exit 1
+    elif [ -z "$GIT_LOCAL_TAG" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but no matching local Git tag was found" >&2
+        exit 1
+    elif [ "$GIT_LOCAL_HEAD" != "$GIT_LOCAL_TAG" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but the matching Git tag is not checked out" >&2
+        exit 1
+    elif [ -z "$GIT_REMOTE_TAG" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but no matching remote Git tag was found" >&2
+        exit 1
+    elif [ "$GIT_LOCAL_TAG" != "$GIT_REMOTE_TAG" ]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but the matching local and remote Git tags differ" >&2
+        exit 1
+    elif [ "$PHP_VERSION_LOCAL" != "$PHP_VERSION" ]; then
+        echo "Unable to build PicoDeprecated: Refusing to build PicoDeprecated with PHP $PHP_VERSION_LOCAL, expecting PHP $PHP_VERSION" >&2
+        exit 1
+    fi
+else
+    if [ -z "$NOCHECK" ] && [[ "$VERSION" != "$BUILD_VERSION"* ]]; then
+        echo "Unable to build PicoDeprecated: Building $VERSION, but PicoDeprecated indicates $BUILD_VERSION" >&2
+        exit 1
+    elif [ -n "$PUBLISH" ]; then
+        echo "Unable to build PicoDeprecated: Refusing to publish a dev version" >&2
+        exit 1
+    fi
+fi
+
+# build in progress...
+APP_DIR="$PWD"
+
+BUILD_DIR="$(mktemp -d)"
+[ -n "$NOCLEAN" ] || trap "rm -rf ${BUILD_DIR@Q}" ERR EXIT
+
+echo "Building Pico $BUILD_VERSION ($VERSION)..."
+[ -z "$NOCLEAN" ] || echo "Build directory: $BUILD_DIR"
+echo
+
+# copy PicoDeprecated for building
+# the working tree is always clean for non-dev versions, see build checks above
+echo "Creating clean working tree copy of '$BUILD_PROJECT'..."
+rsync -a \
+    --exclude="/.build" \
+    --exclude="/.git" \
+    --exclude="/.github" \
+    --exclude="/.gitattributes" \
+    --exclude="/.gitignore" \
+    --exclude="/.phpcs.xml" \
+    --exclude-from=<(git ls-files --exclude-standard -oi --directory) \
+    ./ "$BUILD_DIR/"
+
+echo
+
+# switch to build dir...
+cd "$BUILD_DIR"
+
+# remove picocms/pico dependency
+echo "Removing '$PICO_PROJECT' dependency..."
+"$COMPOSER" remove --no-update "$PICO_PROJECT"
+echo
+
+# set minimum stability
+if [ "$VERSION_STABILITY" != "stable" ]; then
+    echo "Setting minimum stability to '$VERSION_STABILITY'..."
+    "$COMPOSER" config "minimum-stability" "$VERSION_STABILITY"
+    "$COMPOSER" config "prefer-stable" "true"
+    echo
+fi
+
+# install dependencies
+echo "Installing Composer dependencies..."
+"$COMPOSER" install --no-dev --optimize-autoloader
+echo
+
+# prepare release
+echo "Removing '.git' directory..."
+rm -rf .git
+
+echo "Removing '.git' directories of dependencies..."
+find vendor/ -type d -path 'vendor/*/*/.git' -print0 | xargs -0 rm -rf
+
+echo
+
+# restore picocms/pico dependency
+echo "Restoring '$PICO_PROJECT' dependency..."
+"$COMPOSER" require --no-update "$PICO_PROJECT self.version"
+echo
+
+# create release archives
+ARCHIVE_FILENAME="$BUILD_NAME-release-$VERSION"
+
+echo "Creating release archive '$ARCHIVE_FILENAME.tar.gz'..."
+find . -mindepth 1 -maxdepth 1 -printf '%f\0' \
+    | xargs -0 -- tar -czf "$APP_DIR/$ARCHIVE_FILENAME.tar.gz" --
+
+echo "Creating release archive '$ARCHIVE_FILENAME.zip'..."
+zip -q -r "$APP_DIR/$ARCHIVE_FILENAME.zip" .
+
+# publish release
+if [ -n "$PUBLISH" ]; then
+    echo
+
+    # switch to app dir
+    cd "$APP_DIR"
+
+    # create GitHub release and upload release archives
+    echo "Creating GitHub release and uploading release archives..."
+
+    GITHUB_PRERELEASE=
+    [ "$VERSION_STABILITY" == "stable" ] || GITHUB_PRERELEASE="--prerelease"
+
+    gh release create "$VERSION" \
+        --title "Version $VERSION_FULL" \
+        --generate-notes \
+        "$GITHUB_PRERELEASE" \
+        "$APP_DIR/$ARCHIVE_FILENAME.tar.gz" \
+        "$APP_DIR/$ARCHIVE_FILENAME.zip"
+fi
diff --git a/.build/clean.sh b/.build/clean.sh
deleted file mode 100755
index 0c44328..0000000
--- a/.build/clean.sh
+++ /dev/null
@@ -1,21 +0,0 @@
-#!/usr/bin/env bash
-set -e
-
-[ -n "$PICO_BUILD_ENV" ] || { echo "No Pico build environment specified" >&2; exit 1; }
-
-# parameters
-ARCHIVE_DIR="${1:-$PICO_PROJECT_DIR}"   # directory to create release archives in
-
-# print parameters
-echo "Cleaning up build environment..."
-printf 'PICO_BUILD_DIR="%s"\n' "$PICO_BUILD_DIR"
-printf 'ARCHIVE_DIR="%s"\n' "$ARCHIVE_DIR"
-echo
-
-echo "Removing build directory..."
-[ ! -d "$PICO_BUILD_DIR" ] || rm -rf "$PICO_BUILD_DIR"
-
-echo "Removing release archives..."
-find "$ARCHIVE_DIR" -mindepth 1 -maxdepth 1 \
-    \( -name 'pico-deprecated-release-*.tar.gz' -o -name 'pico-deprecated-release-*.zip' \) \
-    -delete
diff --git a/.build/init.sh.inc b/.build/init.sh.inc
deleted file mode 100644
index 80828e7..0000000
--- a/.build/init.sh.inc
+++ /dev/null
@@ -1,7 +0,0 @@
-if [ -z "$PICO_BUILD_ENV" ]; then
-    echo "No Pico build environment specified" >&2
-    exit 1
-fi
-
-# add project build dir to $PATH
-export PATH="$PICO_PROJECT_DIR/.build:$PATH"
diff --git a/.build/install.sh b/.build/install.sh
deleted file mode 100755
index 59b89bb..0000000
--- a/.build/install.sh
+++ /dev/null
@@ -1,11 +0,0 @@
-#!/usr/bin/env bash
-set -e
-
-[ -n "$PICO_BUILD_ENV" ] || { echo "No Pico build environment specified" >&2; exit 1; }
-
-# setup build system
-"$PICO_TOOLS_DIR/setup/$PICO_BUILD_ENV.sh" --phpcs
-
-# install dependencies
-echo "Running \`composer install\`..."
-composer install --no-suggest
diff --git a/.build/release.sh b/.build/release.sh
deleted file mode 100755
index be64be3..0000000
--- a/.build/release.sh
+++ /dev/null
@@ -1,49 +0,0 @@
-#!/usr/bin/env bash
-set -e
-
-[ -n "$PICO_BUILD_ENV" ] || { echo "No Pico build environment specified" >&2; exit 1; }
-
-# parameters
-VERSION="${1:-$PROJECT_REPO_TAG}"       # version to create a release for
-ARCHIVE_DIR="${2:-$PICO_PROJECT_DIR}"   # directory to create release archives in
-
-# print parameters
-echo "Creating new release..."
-printf 'VERSION="%s"\n' "$VERSION"
-echo
-
-# set archive name
-if [ -n "$VERSION" ]; then
-    ARCHIVE_NAME="pico-deprecated-release-v${VERSION#v}"
-else
-    ARCHIVE_NAME="pico-deprecated-release-dev-${PROJECT_REPO_BRANCH:-master}"
-fi
-
-# copy project
-rsync -a \
-    --exclude="/.git" \
-    --exclude="/.build" \
-    --exclude="/.github" \
-    --exclude="/.gitattributes" \
-    --exclude="/.gitignore" \
-    --exclude="/.phpcs.xml" \
-    --exclude="/.travis.yml" \
-    --exclude="/pico-deprecated-release-v*.tar.gz" \
-    --exclude="/pico-deprecated-release-v*.zip" \
-    "$PICO_PROJECT_DIR/" \
-    "$PICO_BUILD_DIR/"
-
-cd "$PICO_BUILD_DIR"
-
-# remove picocms/Pico dependency
-echo "Removing 'picocms/pico' dependency..."
-composer remove --no-update "picocms/pico"
-echo
-
-# install dependencies
-echo "Running \`composer install\`..."
-composer install --no-suggest --prefer-dist --no-dev --optimize-autoloader
-echo
-
-# create release archives
-create-release.sh "$PICO_BUILD_DIR" "$ARCHIVE_DIR" "$ARCHIVE_NAME"
diff --git a/.gitattributes b/.gitattributes
index 0eacfff..1475def 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,4 +3,3 @@
 /.gitattributes export-ignore
 /.gitignore     export-ignore
 /.phpcs.xml     export-ignore
-/.travis.yml    export-ignore
diff --git a/.github/workflows/stale.yml b/.github/workflows/stale.yml
index 878d942..5ca512e 100644
--- a/.github/workflows/stale.yml
+++ b/.github/workflows/stale.yml
@@ -1,28 +1,33 @@
-name: "Mark or close stale issues and PRs"
+name: Mark or close stale issues and PRs
+
 on:
-  schedule:
-    - cron: "0 12 * * *"
+    schedule:
+        - cron: "0 */6 * * *"
 
 jobs:
-  stale:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/stale@v3
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          days-before-stale: 7
-          days-before-close: 2
-          stale-issue-message: >
-            This issue has been automatically marked as stale because it has not had
-            recent activity. It will be closed in two days if no further activity
-            occurs. Thank you for your contributions! :+1:
-          stale-pr-message: >
-            This pull request has been automatically marked as stale because it has not had
-            recent activity. It will be closed in two days if no further activity
-            occurs. Thank you for your contributions! :+1:
-          stale-pr-label: "info: Stale"
-          stale-issue-label: "info: Stale"
-          exempt-issue-labels: "type: Bug,type: Enhancement,type: Feature,type: Idea,type: Release,info: Pinned"
-          exempt-pr-labels: "type: Bug,type: Enhancement,type: Feature,type: Idea,type: Release,info: Pinned"
-          remove-stale-when-updated: true
+    stale:
+        name: Mark or close stale issues and PRs
+
+        runs-on: ubuntu-latest
+        permissions:
+            issues: write
+            pull-requests: write
 
+        steps:
+            -   uses: actions/stale@v3
+                with:
+                    days-before-stale: 7
+                    days-before-close: 2
+                    stale-issue-message: >
+                        This issue has been automatically marked as stale because it has not had
+                        recent activity. It will be closed in two days if no further activity
+                        occurs. Thank you for your contributions! :+1:
+                    stale-pr-message: >
+                        This pull request has been automatically marked as stale because it has not had
+                        recent activity. It will be closed in two days if no further activity
+                        occurs. Thank you for your contributions! :+1:
+                    stale-pr-label: "info: Stale"
+                    stale-issue-label: "info: Stale"
+                    exempt-issue-labels: "type: Bug,type: Enhancement,type: Feature,type: Idea,type: Release,info: Pinned"
+                    exempt-pr-labels: "type: Bug,type: Enhancement,type: Feature,type: Idea,type: Release,info: Pinned"
+                    remove-stale-when-updated: true
diff --git a/.github/workflows/test-pr.yml b/.github/workflows/test-pr.yml
new file mode 100644
index 0000000..c6fd342
--- /dev/null
+++ b/.github/workflows/test-pr.yml
@@ -0,0 +1,27 @@
+name: Test PicoDeprecated pull request
+
+on:
+    pull_request: {}
+
+jobs:
+    test:
+        name: Test PicoDeprecated
+        uses: ./.github/workflows/test.yml
+
+    bouncer:
+        name: Bouncer
+
+        needs: test
+        if: ${{ always() }}
+
+        runs-on: ubuntu-latest
+        permissions: {}
+
+        steps:
+            - name: Check build matrix status
+              if: ${{ needs.test.result != 'success' }}
+              run: |
+                  :
+                  echo "Some tests of PicoDeprecated failed." >&2
+                  echo "Please check the GitHub workflow logs for details." >&2
+                  exit 1
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
new file mode 100644
index 0000000..24b72f2
--- /dev/null
+++ b/.github/workflows/test.yml
@@ -0,0 +1,105 @@
+name: Test PicoDeprecated
+
+on:
+    push:
+        branches:
+            - 'master'
+            - 'pico-3.0'
+        tags: [ 'v*.*.*' ]
+    workflow_call: {}
+
+jobs:
+    test:
+        name: PHP ${{ matrix.PHP_VERSION }}
+
+        runs-on: ubuntu-latest
+        permissions:
+            contents: read
+
+        strategy:
+            matrix:
+                PHP_VERSION:
+                    - '7.2'
+                    - '7.3'
+                    - '7.4'
+                    - '8.0'
+                    - '8.1'
+                    - '8.2'
+                    - '8.3'
+                    - '8.4'
+                    - '8.5'
+                    - 'nightly'
+            fail-fast: false
+
+        continue-on-error: ${{ matrix.PHP_VERSION == 'nightly' }}
+
+        env:
+            PHP_VERSION: ${{ matrix.PHP_VERSION }}
+
+        steps:
+            -   name: Checkout repository
+                uses: actions/checkout@v6
+
+            -   name: Setup PHP ${{ env.PHP_VERSION }}
+                uses: shivammathur/setup-php@v2
+                with:
+                    php-version: ${{ env.PHP_VERSION }}
+                    tools: composer, phpcs
+
+            -   name: Setup Composer auth
+                run: |
+                    composer config --global github-oauth.github.com "${{ github.token }}"
+
+            -   name: Get Composer cache directory
+                run: |
+                    COMPOSER_CACHE_DIR="$(composer config --global cache-dir)"
+                    echo "COMPOSER_CACHE_DIR=$COMPOSER_CACHE_DIR" | tee -a "$GITHUB_ENV"
+
+            -   name: Restore Composer cache
+                uses: actions/cache@v5
+                with:
+                    path: ${{ env.COMPOSER_CACHE_DIR }}
+                    key: ${{ runner.os }}-composer-php${{ env.PHP_VERSION }}
+                    restore-keys: |
+                        ${{ runner.os }}-composer-
+
+            -   name: Setup Composer root version
+                if: ${{ github.ref_type == 'branch' }}
+                run: |
+                    COMPOSER_ROOT_VERSION=
+
+                    COMPOSER_ROOT_VERSION="$(composer config \
+                        extra.branch-alias."dev-${GITHUB_HEAD_REF:-$GITHUB_REF_NAME}" \
+                        2> /dev/null || true)"
+
+                    if [ -z "$COMPOSER_ROOT_VERSION" ]; then
+                        COMPOSER_ROOT_VERSION="$(php -r "
+                            class AbstractPicoPlugin {}
+                            require('./PicoDeprecated.php');
+                            echo preg_replace('/\.[0-9]+-dev$/', '.x-dev', PicoDeprecated::VERSION);
+                        ")"
+                    fi
+
+                    echo "COMPOSER_ROOT_VERSION=$COMPOSER_ROOT_VERSION" | tee -a "$GITHUB_ENV"
+
+            -   name: Install Composer dependencies
+                run: |
+                    composer install
+
+            -   name: Read PicoDeprecated version
+                run: |
+                    VERSION="$(php -r "
+                        class AbstractPicoPlugin {}
+                        require('./PicoDeprecated.php');
+                        echo PicoDeprecated::VERSION;
+                    ")"
+                    echo "VERSION=$VERSION" | tee -a "$GITHUB_ENV"
+
+            -   name: Check PicoDeprecated version
+                if: ${{ github.ref_type == 'tag' }}
+                run: |
+                    [ "$GITHUB_REF_NAME" == "v$VERSION" ]
+
+            -   name: Run PHP_CodeSniffer
+                run: |
+                    phpcs --standard=.phpcs.xml
diff --git a/.gitignore b/.gitignore
index 70ac336..e9e1e3e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,8 +15,5 @@ desktop.ini
 /vendor
 
 # Build system
-/.build/build
-/.build/deploy
-/.build/ci-tools
 /pico-deprecated-release-*.tar.gz
 /pico-deprecated-release-*.zip
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index e521bf8..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,65 +0,0 @@
-dist: trusty
-sudo: false
-
-language: php
-
-cache:
-  directories:
-    - $HOME/.composer/cache/files
-
-jobs:
-  include:
-    # Test stage
-    - php: 5.3
-      dist: precise
-    - php: 5.4
-    - php: 5.5
-    - php: 5.6
-    - php: 7.0
-    - php: 7.1
-    - php: 7.2
-    - php: 7.3
-    - php: 7.4
-    - php: nightly
-    - php: hhvm-3.27 # until Sep 2019
-    - php: hhvm-3.30 # until Nov 2019
-
-    # Release deployment stage
-    - stage: deploy-release
-      if: tag IS present
-      php: 5.3
-      dist: precise
-      install:
-        - install.sh
-      script:
-        - 'true'
-      before_deploy:
-        - release.sh "$PROJECT_REPO_TAG"
-      deploy:
-        provider: releases
-        api_key: ${GITHUB_OAUTH_TOKEN}
-        file:
-          - pico-deprecated-release-$PROJECT_REPO_TAG.tar.gz
-          - pico-deprecated-release-$PROJECT_REPO_TAG.zip
-        skip_cleanup: true
-        name: Version ${PROJECT_REPO_TAG:1}
-        draft: true
-        on:
-          tags: true
-
-  # Ignore nightly build failures
-  allow_failures:
-    - php: nightly
-  fast_finish: true
-
-before_install:
-  - export PICO_TOOLS_DIR="$HOME/__picocms_tools"
-  - git clone --branch="$TOOLS_REPO_BRANCH" "https://github.com/$TOOLS_REPO_SLUG.git" "$PICO_TOOLS_DIR"
-  - . "$PICO_TOOLS_DIR/init/travis.sh.inc"
-  - . "$PICO_PROJECT_DIR/.build/init.sh.inc"
-
-install:
-  - install.sh
-
-script:
-  - phpcs --standard=.phpcs.xml "$PICO_PROJECT_DIR"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4cc5d1d..9ec2a00 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,28 @@ Pico Deprecated Changelog
           because they are already listed in Pico's changelog. Only functional
           changes and/or BC-breaking changes are listed below.
 
+### Version 3.0.0-beta.1
+Released: -
+
+```
+* [New] Update to API version 4 (no functional changes necessary)
+* [New] Add new continous integration (CI) pipeline using GitHub Actions
+* [New] Add new build script and Makefile to simplify PicoDeprecated's build
+        and release process; see Pico's `CONTRIBUTING.md` for details
+* [New] Add `PicoDeprecated::VERSION` and `PicoDeprecated::VERSION_ID`
+        constants (similar to Pico's matching constants)
+* [New] Add plugin and theme compatibility plugin for API version 3
+* [Changed] Enable PHP strict typing
+* [Changed] ! Move classes to `\picocms\PicoDeprecated\…` PHP namespace
+* [Fixed] Don't instantiate compatibility plugins multiple times
+* [Fixed] Update Twig API usage to be compatible with Twig 3.3
+```
+
+### Version 3.0.0-alpha.2
+Released: 2020-12-24
+
+No changes
+
 ### Version 2.1.4
 Released: 2020-08-29
 
@@ -36,6 +58,13 @@ Released: 2020-04-10
 
 No changes
 
+### Version 3.0.0-alpha.1
+Released: 2020-03-29
+
+```
+* [New] Kick-start development of Pico 3.0
+```
+
 ### Version 2.1.1
 Released: 2019-12-31
 
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..82a1952
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,55 @@
+# PicoDeprecated -- Makefile
+# Makefile to build new PicoDeprecated releases using './.build/build.sh'.
+#
+# This file is part of Pico, a stupidly simple, blazing fast, flat file CMS.
+# Visit us at https://picocms.org/ for more info.
+#
+# Copyright (c) 2022  Daniel Rudolf <https://www.daniel-rudolf.de>
+#
+# This work is licensed under the terms of the MIT license.
+# For a copy, see LICENSE file or <https://opensource.org/licenses/MIT>.
+#
+# SPDX-License-Identifier: MIT
+# License-Filename: LICENSE
+
+version?=
+nocheck?=false
+publish=false
+
+php?=php
+composer?=composer
+
+app_name=pico-deprecated
+archive_tar=$(app_name)-release-*.tar.gz
+archive_zip=$(app_name)-release-*.zip
+export=$(app_name)-export.tar.gz
+
+all: clean build
+
+clean: clean-build clean-export
+
+clean-build:
+	find "." -name "$(archive_tar)" -exec rm -f {} \;
+	find "." -name "$(archive_zip)" -exec rm -f {} \;
+
+clean-export:
+	rm -f "./$(export)"
+
+build: export PHP=$(php)
+build: export COMPOSER=$(composer)
+build:
+	./.build/build.sh$(if $(filter true,$(publish)), --publish,)$(if $(filter true,$(nocheck)), --no-check,)$(if $(version), "$(version)",)
+
+export:
+	git archive --prefix "$(app_name)/" -o "./$(export)" HEAD
+
+publish: publish=true
+publish: build
+
+composer:
+	$(composer) install --optimize-autoloader --no-dev
+
+.PHONY: all \
+	clean clean-build clean-export \
+	build export publish \
+	composer
diff --git a/PicoDeprecated.php b/PicoDeprecated.php
index 9334661..fea86a9 100644
--- a/PicoDeprecated.php
+++ b/PicoDeprecated.php
@@ -16,6 +16,12 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+use picocms\PicoDeprecated\Plugin\MainPlugin;
+use picocms\PicoDeprecated\PluginApiPluginInterface;
+use picocms\PicoDeprecated\PluginInterface;
+
 /**
  * Maintain backward compatibility to older Pico releases
  *
@@ -28,67 +34,88 @@
  * API versions than the one of Pico's core, even when there was just the
  * slightest change.
  *
- * {@see http://picocms.org/plugins/deprecated/} for a full list of features.
+ * {@see https://picocms.org/plugins/deprecated/} for a full list of features.
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
 class PicoDeprecated extends AbstractPicoPlugin
 {
+    /**
+     * PicoDeprecated version
+     *
+     * @var string
+     */
+    public const VERSION = '3.0.0-dev';
+
+    /**
+     * PicoDeprecated version ID
+     *
+     * @var int
+     */
+    public const VERSION_ID = 30000;
+
     /**
      * API version used by this plugin
      *
      * @var int
      */
-    const API_VERSION = 3;
+    public const API_VERSION = 4;
 
     /**
      * API version 0, used by Pico 0.9 and earlier
      *
      * @var int
      */
-    const API_VERSION_0 = 0;
+    public const API_VERSION_0 = 0;
 
     /**
      * API version 1, used by Pico 1.0
      *
      * @var int
      */
-    const API_VERSION_1 = 1;
+    public const API_VERSION_1 = 1;
 
     /**
      * API version 2, used by Pico 2.0
      *
      * @var int
      */
-    const API_VERSION_2 = 2;
+    public const API_VERSION_2 = 2;
 
     /**
      * API version 3, used by Pico 2.1
      *
      * @var int
      */
-    const API_VERSION_3 = 3;
+    public const API_VERSION_3 = 3;
+
+    /**
+     * API version 4, used by Pico 3.0
+     *
+     * @var int
+     */
+    public const API_VERSION_4 = 4;
 
     /**
      * Loaded plugins, indexed by API version
      *
      * @see PicoDeprecated::getPlugins()
      *
-     * @var object[]
+     * @var object[][]
      */
-    protected $plugins = array();
+    protected $plugins = [];
 
     /**
      * Loaded compatibility plugins
      *
      * @see PicoDeprecated::getCompatPlugins()
      *
-     * @var PicoCompatPluginInterface[]
+     * @var PluginInterface[]
      */
-    protected $compatPlugins = array();
+    protected $compatPlugins = [];
 
     /**
      * {@inheritDoc}
@@ -101,18 +128,24 @@ public function __construct(Pico $pico)
             require(__DIR__ . '/vendor/autoload.php');
         }
 
-        if (!class_exists('PicoMainCompatPlugin')) {
-            die(
-                "Cannot find PicoDeprecated's 'vendor/autoload.php'. If you're using a composer-based Pico install, "
+        if (!class_exists(MainPlugin::class)) {
+            throw new RuntimeException(
+                "Cannot find PicoDeprecated's 'vendor/autoload.php'. If you're using a Composer-based Pico install, "
                 . "run `composer update`. If you're rather trying to use one of PicoDeprecated's pre-built release "
                 . "packages, make sure to download PicoDeprecated's release package matching Pico's version named "
-                . "'pico-deprecated-release-v*.tar.gz' (don't download a source code package)."
+                . "'pico-deprecated-release-v*.tar.gz'; make sure not to download a source code package."
             );
         }
 
-        if ($pico::API_VERSION !== static::API_VERSION) {
+        if ($pico::VERSION_ID !== self::VERSION_ID) {
             throw new RuntimeException(
-                'PicoDeprecated requires API version ' . static::API_VERSION . ', '
+                'PicoDeprecated v' . self::VERSION . ' is incompatible with Pico v' . $pico::VERSION
+            );
+        }
+
+        if ($pico::API_VERSION !== self::API_VERSION) {
+            throw new RuntimeException(
+                'PicoDeprecated requires API version ' . self::API_VERSION . ', '
                 . 'but Pico is running API version ' . $pico::API_VERSION
             );
         }
@@ -130,10 +163,10 @@ public function handleEvent($eventName, array $params)
             $isCoreEvent = in_array($eventName, $this->getCoreEvents());
             foreach ($this->compatPlugins as $plugin) {
                 if ($isCoreEvent) {
-                    if ($plugin->getApiVersion() === static::API_VERSION) {
+                    if ($plugin->getApiVersion() === self::API_VERSION) {
                         $plugin->handleEvent($eventName, $params);
                     }
-                } elseif ($plugin instanceof PicoPluginApiCompatPluginInterface) {
+                } elseif ($plugin instanceof PluginApiPluginInterface) {
                     $plugin->handleCustomEvent($eventName, $params);
                 }
             }
@@ -148,15 +181,15 @@ public function handleEvent($eventName, array $params)
      *
      * @param object[] $plugins loaded plugin instances
      */
-    public function onPluginsLoaded(array $plugins)
+    public function onPluginsLoaded(array $plugins): void
     {
-        $this->loadCompatPlugin('PicoMainCompatPlugin');
+        $this->loadCompatPlugin(MainPlugin::class);
 
         foreach ($plugins as $plugin) {
             $this->loadPlugin($plugin);
         }
 
-        $this->getPico()->triggerEvent('onPicoDeprecated', array($this));
+        $this->getPico()->triggerEvent('onPicoDeprecated', [ $this ]);
     }
 
     /**
@@ -167,7 +200,7 @@ public function onPluginsLoaded(array $plugins)
      *
      * @param object $plugin loaded plugin instance
      */
-    public function onPluginManuallyLoaded($plugin)
+    public function onPluginManuallyLoaded(object $plugin): void
     {
         $this->loadPlugin($plugin);
     }
@@ -179,7 +212,7 @@ public function onPluginManuallyLoaded($plugin)
      * @param int    $themeApiVersion API version of the theme
      * @param array  $themeConfig     config array of the theme
      */
-    public function onThemeLoaded($theme, $themeApiVersion, array &$themeConfig)
+    public function onThemeLoaded(string $theme, int $themeApiVersion, array &$themeConfig): void
     {
         $this->loadThemeApiCompatPlugin($themeApiVersion);
     }
@@ -193,13 +226,13 @@ public function onThemeLoaded($theme, $themeApiVersion, array &$themeConfig)
      *
      * @param object $plugin loaded plugin instance
      */
-    protected function loadPlugin($plugin)
+    protected function loadPlugin(object $plugin): void
     {
         $pluginName = get_class($plugin);
 
         $apiVersion = $this->getPluginApiVersion($plugin);
         if (!isset($this->plugins[$apiVersion])) {
-            $this->plugins[$apiVersion] = array();
+            $this->plugins[$apiVersion] = [];
             $this->loadPluginApiCompatPlugin($apiVersion);
         }
 
@@ -213,23 +246,27 @@ protected function loadPlugin($plugin)
      *
      * @return object[] loaded plugin instances
      */
-    public function getPlugins($apiVersion)
+    public function getPlugins(int $apiVersion): array
     {
-        return isset($this->plugins[$apiVersion]) ? $this->plugins[$apiVersion] : array();
+        return $this->plugins[$apiVersion] ?? [];
     }
 
     /**
      * Loads a compatibility plugin
      *
-     * @param PicoCompatPluginInterface|string $plugin either the class name of
-     *     a plugin to instantiate or a plugin instance
+     * @param PluginInterface|string $plugin either the class name of a plugin
+     *     to instantiate or a plugin instance
      *
-     * @return PicoCompatPluginInterface instance of the loaded plugin
+     * @return PluginInterface instance of the loaded plugin
      */
-    public function loadCompatPlugin($plugin)
+    public function loadCompatPlugin($plugin): PluginInterface
     {
         if (!is_object($plugin)) {
             $className = (string) $plugin;
+            if (isset($this->compatPlugins[$className])) {
+                return $this->compatPlugins[$className];
+            }
+
             if (class_exists($className)) {
                 $plugin = new $className($this->getPico(), $this);
             } else {
@@ -244,10 +281,10 @@ public function loadCompatPlugin($plugin)
             return $this->compatPlugins[$className];
         }
 
-        if (!($plugin instanceof PicoCompatPluginInterface)) {
+        if (!($plugin instanceof PluginInterface)) {
             throw new RuntimeException(
                 "Unable to load PicoDeprecated compatibility plugin '" . $className . "': "
-                . "Compatibility plugins must implement 'PicoCompatPluginInterface'"
+                . "Compatibility plugins must implement '" . PluginInterface::class . "'"
             );
         }
 
@@ -269,10 +306,10 @@ public function loadCompatPlugin($plugin)
      *
      * @param int $apiVersion API version to load the compatibility plugin for
      */
-    protected function loadPluginApiCompatPlugin($apiVersion)
+    protected function loadPluginApiCompatPlugin(int $apiVersion): void
     {
-        if ($apiVersion !== static::API_VERSION) {
-            $this->loadCompatPlugin('PicoPluginApi' . $apiVersion . 'CompatPlugin');
+        if ($apiVersion !== self::API_VERSION) {
+            $this->loadCompatPlugin('picocms\PicoDeprecated\Plugin\PluginApi' . $apiVersion . 'Plugin');
         }
     }
 
@@ -281,19 +318,19 @@ protected function loadPluginApiCompatPlugin($apiVersion)
      *
      * @param int $apiVersion API version to load the compatibility plugin for
      */
-    protected function loadThemeApiCompatPlugin($apiVersion)
+    protected function loadThemeApiCompatPlugin(int $apiVersion): void
     {
-        if ($apiVersion !== static::API_VERSION) {
-            $this->loadCompatPlugin('PicoThemeApi' . $apiVersion . 'CompatPlugin');
+        if ($apiVersion !== self::API_VERSION) {
+            $this->loadCompatPlugin('picocms\PicoDeprecated\Plugin\ThemeApi' . $apiVersion . 'Plugin');
         }
     }
 
     /**
      * Returns all loaded compatibility plugins
      *
-     * @return PicoCompatPluginInterface[] list of loaded compatibility plugins
+     * @return PluginInterface[] list of loaded compatibility plugins
      */
-    public function getCompatPlugins()
+    public function getCompatPlugins(): array
     {
         return $this->compatPlugins;
     }
@@ -313,7 +350,7 @@ public function getCompatPlugins()
      * @param string $eventName  event to trigger
      * @param array  $params     optional parameters to pass
      */
-    public function triggerEvent($apiVersion, $eventName, array $params = array())
+    public function triggerEvent(int $apiVersion, string $eventName, array $params = []): void
     {
         foreach ($this->getPlugins($apiVersion) as $plugin) {
             $plugin->handleEvent($eventName, $params);
@@ -327,7 +364,7 @@ public function triggerEvent($apiVersion, $eventName, array $params = array())
      *
      * @return int API version used by the plugin
      */
-    public function getPluginApiVersion($plugin)
+    public function getPluginApiVersion(object $plugin): int
     {
         $pluginApiVersion = self::API_VERSION_0;
         if ($plugin instanceof PicoPluginInterface) {
@@ -345,9 +382,9 @@ public function getPluginApiVersion($plugin)
      *
      * @return string[] list of Pico's core events
      */
-    public function getCoreEvents()
+    public function getCoreEvents(): array
     {
-        return array(
+        return [
             'onPluginsLoaded',
             'onPluginManuallyLoaded',
             'onConfigLoaded',
@@ -377,7 +414,7 @@ public function getCoreEvents()
             'onMetaHeaders',
             'onYamlParserRegistered',
             'onParsedownRegistered',
-            'onTwigRegistered'
-        );
+            'onTwigRegistered',
+        ];
     }
 }
diff --git a/README.md b/README.md
index 4c3091a..7378db9 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@ Pico Deprecated Plugin
 
 This is the repository of Pico's official `PicoDeprecated` plugin.
 
-Pico is a stupidly simple, blazing fast, flat file CMS. See http://picocms.org/ for more info.
+Pico is a stupidly simple, blazing fast, flat file CMS. See https://picocms.org/ for more info.
 
 `PicoDeprecated`'s purpose is to maintain backward compatibility to older versions of Pico, by re-introducing characteristics that were removed from Pico's core. It for example triggers old events (like the `before_render` event used before Pico 1.0) and reads config files that were written in PHP (`config/config.php`, used before Pico 2.0).
 
@@ -27,9 +27,9 @@ Usage
 
 You can explicitly disable `PicoDeprecated` by adding `PicoDeprecated.enabled: false` to your `config/config.yml`. If you want to re-enable `PicoDeprecated`, simply remove this line from your `config/config.yml`. `PicoDeprecated` itself has no configuration options, it enables and disables all of its features depending on whether there are plugins and/or themes requiring said characteristics.
 
-`PicoDeprecated`'s functionality is split into various so-called "compatibility plugins". There are compatibility plugins for every old API version (Pico 0.9 and earlier were using API version 0, Pico 1.0 was using API version 1 and Pico 2.0 was using API version 2; the current API version is version 3, used by Pico 2.1), one for plugins and another one for themes. Their purpose is to re-introduce characteristics plugins and themes using said API version might rely on. For example, plugin API compatibility plugins are responsible for simulating old Pico core events (like the `before_render` event used by Pico 0.9 and earlier). Theme API compatibility plugins will e.g. register old Twig variables (like the `is_front_page` Twig variable used by Pico 1.0). If you install a plugin using API version 2, the corresponding `PicoPluginApi2CompatPlugin` will be loaded. All plugin API compatibility plugins also depend on their theme counterpart, thus `PicoThemeApi2CompatPlugin` will be loaded, too. Furthermore all compatibility plugins depend on their respective API successors.
+`PicoDeprecated`'s functionality is split into various so-called "compatibility plugins". There are compatibility plugins for every old API version (Pico 0.9 and earlier were using API version 0, Pico 1.0 was using API version 1 and Pico 2.0 was using API version 2; the current API version is version 3, used by Pico 2.1), one for plugins and another one for themes. Their purpose is to re-introduce characteristics plugins and themes using said API version might rely on. For example, plugin API compatibility plugins are responsible for simulating old Pico core events (like the `before_render` event used by Pico 0.9 and earlier). Theme API compatibility plugins will e.g. register old Twig variables (like the `is_front_page` Twig variable used by Pico 1.0). If you install a plugin using API version 2, the corresponding `picocms\PicoDeprecated\Plugin\PluginApi2Plugin` will be loaded. All plugin API compatibility plugins also depend on their theme counterpart, thus `picocms\PicoDeprecated\Plugin\ThemeApi2Plugin` will be loaded, too. Furthermore all compatibility plugins depend on their respective API successors.
 
-The plugin exposes a simple API to allow other plugins to load their own compatibility plugins. As a plugin developer you may use the `PicoDeprecated::loadCompatPlugin(PicoCompatPluginInterface $compatPlugin)` method to load a custom compatibility plugin. Use `PicoDeprecated::getCompatPlugins()` to return a list of all loaded compatibility plugins. You can furthermore use the `PicoDeprecated::getPlugins(int $apiVersion)` method to return a list of all loaded Pico plugins using a particular API version. If you want to trigger a custom event on plugins using a particular API version only, use `PicoDeprecated::triggerEvent(int $apiVersion, string $eventName, array $parameters = [])`. `PicoDeprecated` furthermore triggers the custom `onPicoDeprecated(PicoDeprecated $picoDeprecated)` event.
+The plugin exposes a simple API to allow other plugins to load their own compatibility plugins. As a plugin developer you may use the `PicoDeprecated::loadCompatPlugin(picocms\PicoDeprecated\PluginInterface $compatPlugin)` method to load a custom compatibility plugin. Use `PicoDeprecated::getCompatPlugins()` to return a list of all loaded compatibility plugins. You can furthermore use the `PicoDeprecated::getPlugins(int $apiVersion)` method to return a list of all loaded Pico plugins using a particular API version. If you want to trigger a custom event on plugins using a particular API version only, use `PicoDeprecated::triggerEvent(int $apiVersion, string $eventName, array $parameters = [])`. `PicoDeprecated` furthermore triggers the custom `onPicoDeprecated(PicoDeprecated $picoDeprecated)` event.
 
 Getting Help
 ------------
diff --git a/composer.json b/composer.json
index daf56bc..6e894d8 100644
--- a/composer.json
+++ b/composer.json
@@ -3,7 +3,7 @@
     "type": "pico-plugin",
     "description": "This is Pico's official PicoDeprecated plugin. Pico is a stupidly simple, blazing fast, flat file CMS.",
     "keywords": [ "pico", "picocms", "picocms-plugin", "pico-deprecated", "compatibility", "deprecation" ],
-    "homepage": "http://picocms.org/",
+    "homepage": "https://picocms.org/",
     "license": "MIT",
     "authors": [
         {
@@ -13,7 +13,7 @@
         },
         {
             "name": "The Pico Community",
-            "homepage": "http://picocms.org/"
+            "homepage": "https://picocms.org/"
         },
         {
             "name": "Contributors",
@@ -21,16 +21,20 @@
         }
     ],
     "support": {
-        "docs": "http://picocms.org/plugins/deprecated/",
+        "docs": "https://picocms.org/plugins/deprecated/",
         "issues": "https://github.com/picocms/pico-deprecated/issues",
         "source": "https://github.com/picocms/pico-deprecated"
     },
     "require": {
-        "php": ">=5.3.0",
+        "php": ">=7.2.5",
         "picocms/pico": "self.version"
     },
     "autoload": {
-        "classmap": [ "PicoDeprecated.php", "lib/", "plugins/" ]
+        "psr-0": { "PicoDeprecated": "" },
+        "psr-4": {
+            "picocms\\PicoDeprecated\\": "lib/",
+            "picocms\\PicoDeprecated\\Plugin\\": "plugins/"
+        }
     },
     "extra": {
         "branch-alias": {
diff --git a/lib/AbstractPicoCompatPlugin.php b/lib/AbstractPlugin.php
similarity index 64%
rename from lib/AbstractPicoCompatPlugin.php
rename to lib/AbstractPlugin.php
index 1d3961a..80804f7 100644
--- a/lib/AbstractPicoCompatPlugin.php
+++ b/lib/AbstractPlugin.php
@@ -4,32 +4,39 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/lib/AbstractPicoCompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/lib/AbstractPlugin.php>
  *
  * SPDX-License-Identifier: MIT
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated;
+
+use Pico;
+use PicoDeprecated;
+
 /**
  * Abstract class to extend from when implementing a PicoDeprecated
  * compatibility plugin
  *
- * Please refer to {@see PicoCompatPluginInterface} for more information about
- * how to develop a PicoDeprecated compatibility plugin.
+ * Please refer to {@see PicoPluginInterface} for more information about how to
+ * develop a PicoDeprecated compatibility plugin.
  *
- * @see PicoCompatPluginInterface
+ * @see PicoPluginInterface
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-abstract class AbstractPicoCompatPlugin implements PicoCompatPluginInterface
+abstract class AbstractPlugin implements PluginInterface
 {
     /**
      * Current instance of Pico
      *
-     * @see PicoCompatPluginInterface::getPico()
+     * @see PicoPluginInterface::getPico()
      *
      * @var Pico
      */
@@ -38,7 +45,7 @@ abstract class AbstractPicoCompatPlugin implements PicoCompatPluginInterface
     /**
      * Instance of the main PicoDeprecated plugin
      *
-     * @see PicoCompatPluginInterface::getPicoDeprecated()
+     * @see PluginInterface::getPicoDeprecated()
      *
      * @var PicoDeprecated
      */
@@ -47,11 +54,11 @@ abstract class AbstractPicoCompatPlugin implements PicoCompatPluginInterface
     /**
      * List of plugins which this plugin depends on
      *
-     * @see PicoCompatPluginInterface::getDependencies()
+     * @see PicoPluginInterface::getDependencies()
      *
      * @var string[]
      */
-    protected $dependsOn = array();
+    protected $dependsOn = [];
 
     /**
      * Constructs a new instance of a PicoDeprecated compatibility plugin
@@ -68,17 +75,17 @@ public function __construct(Pico $pico, PicoDeprecated $picoDeprecated)
     /**
      * {@inheritDoc}
      */
-    public function handleEvent($eventName, array $params)
+    public function handleEvent(string $eventName, array $params): void
     {
         if (method_exists($this, $eventName)) {
-            call_user_func_array(array($this, $eventName), $params);
+            call_user_func_array([ $this, $eventName ], $params);
         }
     }
 
     /**
      * {@inheritDoc}
      */
-    public function getPico()
+    public function getPico(): Pico
     {
         return $this->pico;
     }
@@ -86,7 +93,7 @@ public function getPico()
     /**
      * {@inheritDoc}
      */
-    public function getPicoDeprecated()
+    public function getPicoDeprecated(): PicoDeprecated
     {
         return $this->picoDeprecated;
     }
@@ -94,7 +101,7 @@ public function getPicoDeprecated()
     /**
      * {@inheritDoc}
      */
-    public function getDependencies()
+    public function getDependencies(): array
     {
         return (array) $this->dependsOn;
     }
diff --git a/lib/AbstractPicoPluginApiCompatPlugin.php b/lib/AbstractPluginApiPlugin.php
similarity index 70%
rename from lib/AbstractPicoPluginApiCompatPlugin.php
rename to lib/AbstractPluginApiPlugin.php
index af32b2f..96cf1ee 100644
--- a/lib/AbstractPicoPluginApiCompatPlugin.php
+++ b/lib/AbstractPluginApiPlugin.php
@@ -4,42 +4,45 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/lib/AbstractPicoPluginApiCompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/lib/AbstractPluginApiPlugin.php>
  *
  * SPDX-License-Identifier: MIT
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated;
+
 /**
  * Abstract class to extend from when implementing a PicoDeprecated plugin API
  * compatibility plugin
  *
- * Please refer to {@see PicoPluginApiCompatPluginInterface} for more information about
- * how to develop a PicoDeprecated plugin API compatibility plugin.
+ * Please refer to {@see PluginApiPluginInterface} for more information about how to
+ * develop a PicoDeprecated plugin API compatibility plugin.
  *
- * @see     PicoPluginApiCompatPluginInterface
+ * @see     PluginApiPluginInterface
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-abstract class AbstractPicoPluginApiCompatPlugin extends AbstractPicoCompatPlugin implements
-    PicoPluginApiCompatPluginInterface
+abstract class AbstractPluginApiPlugin extends AbstractPlugin implements PluginApiPluginInterface
 {
     /**
      * Map of core events matching event signatures of older API versions
      *
-     * @see AbstractPicoPluginApiCompatPlugin::handleEvent()
+     * @see AbstractPluginApiPlugin::handleEvent()
      *
      * @var array<string,string>
      */
-    protected $eventAliases = array();
+    protected $eventAliases = [];
 
     /**
      * {@inheritDoc}
      */
-    public function handleEvent($eventName, array $params)
+    public function handleEvent(string $eventName, array $params): void
     {
         parent::handleEvent($eventName, $params);
 
@@ -54,7 +57,7 @@ public function handleEvent($eventName, array $params)
     /**
      * {@inheritDoc}
      */
-    public function handleCustomEvent($eventName, array $params = array())
+    public function handleCustomEvent(string $eventName, array $params = []): void
     {
         $this->getPicoDeprecated()->triggerEvent($this->getApiVersionSupport(), $eventName, $params);
     }
@@ -65,7 +68,7 @@ public function handleCustomEvent($eventName, array $params = array())
      * @param string $eventName name of the event to trigger
      * @param array  $params    optional parameters to pass
      */
-    protected function triggerEvent($eventName, array $params = array())
+    protected function triggerEvent(string $eventName, array $params = []): void
     {
         $apiVersion = $this->getApiVersionSupport();
         $picoDeprecated = $this->getPicoDeprecated();
diff --git a/lib/PicoPluginApiCompatPluginInterface.php b/lib/PluginApiPluginInterface.php
similarity index 65%
rename from lib/PicoPluginApiCompatPluginInterface.php
rename to lib/PluginApiPluginInterface.php
index 5e71768..f2f9787 100644
--- a/lib/PicoPluginApiCompatPluginInterface.php
+++ b/lib/PluginApiPluginInterface.php
@@ -4,21 +4,25 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/lib/PicoPluginApiCompatPluginInterface.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/lib/PluginApiPluginInterface.php>
  *
  * SPDX-License-Identifier: MIT
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated;
+
 /**
  * Common interface for PicoDeprecated plugin API compatibility plugins
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-interface PicoPluginApiCompatPluginInterface extends PicoCompatPluginInterface
+interface PluginApiPluginInterface extends PluginInterface
 {
     /**
      * Handles custom events for plugins of the supported API version
@@ -26,12 +30,12 @@ interface PicoPluginApiCompatPluginInterface extends PicoCompatPluginInterface
      * @param string $eventName name of the triggered event
      * @param array  $params    passed parameters
      */
-    public function handleCustomEvent($eventName, array $params = array());
+    public function handleCustomEvent(string $eventName, array $params = []): void;
 
     /**
      * Returns the API version this plugin maintains backward compatibility for
      *
      * @return int
      */
-    public function getApiVersionSupport();
+    public function getApiVersionSupport(): int;
 }
diff --git a/lib/PicoCompatPluginInterface.php b/lib/PluginInterface.php
similarity index 70%
rename from lib/PicoCompatPluginInterface.php
rename to lib/PluginInterface.php
index 6ede84f..cdf751b 100644
--- a/lib/PicoCompatPluginInterface.php
+++ b/lib/PluginInterface.php
@@ -4,21 +4,28 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/lib/PicoCompatPluginInterface.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/lib/PluginInterface.php>
  *
  * SPDX-License-Identifier: MIT
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated;
+
+use Pico;
+use PicoDeprecated;
+
 /**
  * Common interface for PicoDeprecated compatibility plugins
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-interface PicoCompatPluginInterface
+interface PluginInterface
 {
     /**
      * Handles a Pico event
@@ -26,14 +33,14 @@ interface PicoCompatPluginInterface
      * @param string $eventName name of the triggered event
      * @param array  $params    passed parameters
      */
-    public function handleEvent($eventName, array $params);
+    public function handleEvent(string $eventName, array $params): void;
 
     /**
      * Returns a list of names of compat plugins required by this plugin
      *
      * @return string[] required plugins
      */
-    public function getDependencies();
+    public function getDependencies(): array;
 
     /**
      * Returns the plugin's instance of Pico
@@ -42,7 +49,7 @@ public function getDependencies();
      *
      * @return Pico the plugin's instance of Pico
      */
-    public function getPico();
+    public function getPico(): Pico;
 
     /**
      * Returns the plugin's main PicoDeprecated plugin instance
@@ -51,12 +58,12 @@ public function getPico();
      *
      * @return PicoDeprecated the plugin's instance of Pico
      */
-    public function getPicoDeprecated();
+    public function getPicoDeprecated(): PicoDeprecated;
 
     /**
      * Returns the version of the API this plugin uses
      *
      * @return int the API version used by this plugin
      */
-    public function getApiVersion();
+    public function getApiVersion(): int;
 }
diff --git a/plugins/PicoMainCompatPlugin.php b/plugins/MainPlugin.php
similarity index 81%
rename from plugins/PicoMainCompatPlugin.php
rename to plugins/MainPlugin.php
index 683aad9..2919950 100644
--- a/plugins/PicoMainCompatPlugin.php
+++ b/plugins/MainPlugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoMainCompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/MainPlugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,15 +18,23 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPlugin;
+use PicoDeprecated;
+use ReflectionObject;
+
 /**
  * Maintains backward compatibility with older Pico versions
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoMainCompatPlugin extends AbstractPicoCompatPlugin
+class MainPlugin extends AbstractPlugin
 {
     /**
      * Load's config.php from Pico's root and config dir
@@ -36,11 +44,11 @@ class PicoMainCompatPlugin extends AbstractPicoCompatPlugin
      * {@see Pico::loadConfig()}. `onConfigLoaded` is triggered later, thus we
      * use the `onPluginsLoaded` event.
      *
-     * @see PicoMainCompatPlugin::loadScriptedConfig()
+     * @see MainPlugin::loadScriptedConfig()
      *
      * @param object[] $plugins loaded plugin instances
      */
-    public function onPluginsLoaded(array $plugins)
+    public function onPluginsLoaded(array $plugins): void
     {
         // deprecated since Pico 1.0
         if (is_file($this->getPico()->getRootDir() . 'config.php')) {
@@ -62,21 +70,18 @@ public function onPluginsLoaded(array $plugins)
      * solution. Otherwise we'd have to copy all of Pico's code dealing with
      * special config settings (like making paths and URLs absolute).
      *
-     * @see PicoMainCompatPlugin::onConfigLoaded()
+     * @see MainPlugin::onConfigLoaded()
      * @see Pico::loadConfig()
      *
      * @param string $configFile path to the config file to load
      */
-    protected function loadScriptedConfig($configFile)
+    protected function loadScriptedConfig(string $configFile): void
     {
         // scope isolated require()
-        $includeConfigClosure = function ($configFile) {
+        $includeConfigClosure = static function ($configFile) {
             require($configFile);
-            return (isset($config) && is_array($config)) ? $config : array();
+            return (isset($config) && is_array($config)) ? $config : [];
         };
-        if (PHP_VERSION_ID >= 50400) {
-            $includeConfigClosure = $includeConfigClosure->bindTo(null);
-        }
 
         $scriptedConfig = $includeConfigClosure($configFile);
 
@@ -85,7 +90,7 @@ protected function loadScriptedConfig($configFile)
             $picoConfigReflector = $picoReflector->getProperty('config');
             $picoConfigReflector->setAccessible(true);
 
-            $config = $picoConfigReflector->getValue($this->getPico()) ?: array();
+            $config = $picoConfigReflector->getValue($this->getPico()) ?: [];
             $config += $scriptedConfig;
 
             $picoConfigReflector->setValue($this->getPico(), $config);
@@ -95,8 +100,8 @@ protected function loadScriptedConfig($configFile)
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
-        return PicoDeprecated::API_VERSION_3;
+        return PicoDeprecated::API_VERSION_4;
     }
 }
diff --git a/plugins/PicoPluginApi2CompatPlugin.php b/plugins/PicoPluginApi2CompatPlugin.php
deleted file mode 100644
index a9ebbee..0000000
--- a/plugins/PicoPluginApi2CompatPlugin.php
+++ /dev/null
@@ -1,131 +0,0 @@
-<?php
-/**
- * This file is part of Pico. It's copyrighted by the contributors recorded
- * in the version control history of the file, available from the following
- * original location:
- *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoPluginApi2CompatPlugin.php>
- *
- * This file was created by splitting up an original file into multiple files,
- * which in turn was previously part of the project's main repository. The
- * version control history of these files apply accordingly, available from
- * the following original locations:
- *
- * <https://github.com/picocms/pico-deprecated/blob/90ea3d5a9767f1511f165e051dd7ffb8f1b3f92e/PicoDeprecated.php>
- * <https://github.com/picocms/Pico/blob/82a342ba445122182b898a2c1800f03c8d16f18c/plugins/00-PicoDeprecated.php>
- *
- * SPDX-License-Identifier: MIT
- * License-Filename: LICENSE
- */
-
-/**
- * Maintains backward compatibility with plugins using API version 2, written
- * for Pico 2.0
- *
- * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
- */
-class PicoPluginApi2CompatPlugin extends AbstractPicoPluginApiCompatPlugin
-{
-    /**
-     * This plugin extends {@see PicoThemeApi2CompatPlugin}
-     *
-     * @var string[]
-     */
-    protected $dependsOn = array('PicoThemeApi2CompatPlugin');
-
-    /**
-     * Map of core events matching event signatures of older API versions
-     *
-     * @see AbstractPicoPluginApiCompatPlugin::handleEvent()
-     *
-     * @var array<string,string>
-     */
-    protected $eventAliases = array(
-        'onPluginsLoaded'         => array('onPluginsLoaded'),
-        'onPluginManuallyLoaded'  => array('onPluginManuallyLoaded'),
-        'onRequestUrl'            => array('onRequestUrl'),
-        'onRequestFile'           => array('onRequestFile'),
-        'onContentLoading'        => array('onContentLoading'),
-        'on404ContentLoading'     => array('on404ContentLoading'),
-        'on404ContentLoaded'      => array('on404ContentLoaded'),
-        'onContentLoaded'         => array('onContentLoaded'),
-        'onMetaParsing'           => array('onMetaParsing'),
-        'onMetaParsed'            => array('onMetaParsed'),
-        'onContentParsing'        => array('onContentParsing'),
-        'onContentPrepared'       => array('onContentPrepared'),
-        'onContentParsed'         => array('onContentParsed'),
-        'onPagesLoading'          => array('onPagesLoading'),
-        'onSinglePageLoading'     => array('onSinglePageLoading'),
-        'onSinglePageContent'     => array('onSinglePageContent'),
-        'onSinglePageLoaded'      => array('onSinglePageLoaded'),
-        'onPagesDiscovered'       => array('onPagesDiscovered'),
-        'onPagesLoaded'           => array('onPagesLoaded'),
-        'onCurrentPageDiscovered' => array('onCurrentPageDiscovered'),
-        'onPageTreeBuilt'         => array('onPageTreeBuilt'),
-        'onPageRendering'         => array('onPageRendering'),
-        'onPageRendered'          => array('onPageRendered'),
-        'onMetaHeaders'           => array('onMetaHeaders'),
-        'onYamlParserRegistered'  => array('onYamlParserRegistered'),
-        'onParsedownRegistered'   => array('onParsedownRegistered'),
-        'onTwigRegistered'        => array('onTwigRegistered')
-    );
-
-    /**
-     * Pico's config array
-     *
-     * @see Pico::$config
-     * @see PicoPluginApi2CompatPlugin::onConfigLoaded()
-     *
-     * @var array|null
-     */
-    protected $config;
-
-    /**
-     * Sets PicoPluginApi1CompatPlugin::$config and handles the theme_url
-     * config param
-     *
-     * @see PicoPluginApi2CompatPlugin::$config
-     *
-     * @param array $config
-     */
-    public function onConfigLoaded(array &$config)
-    {
-        $this->config = &$config;
-
-        if (!empty($config['theme_url'])) {
-            $config['themes_url'] = $this->getPico()->getAbsoluteUrl($config['theme_url']);
-            $config['theme_url'] = &$config['themes_url'];
-        }
-    }
-
-    /**
-     * Triggers the onConfigLoaded event
-     *
-     * @param string $theme           name of current theme
-     * @param int    $themeApiVersion API version of the theme
-     * @param array  &$themeConfig    config array of the theme
-     */
-    public function onThemeLoaded($theme, $themeApiVersion, array &$themeConfig)
-    {
-        $this->triggerEvent('onConfigLoaded', array(&$this->config));
-    }
-
-    /**
-     * {@inheritDoc}
-     */
-    public function getApiVersion()
-    {
-        return PicoDeprecated::API_VERSION_3;
-    }
-
-    /**
-     * {@inheritDoc}
-     */
-    public function getApiVersionSupport()
-    {
-        return PicoDeprecated::API_VERSION_2;
-    }
-}
diff --git a/plugins/PicoPluginApi0CompatPlugin.php b/plugins/PluginApi0Plugin.php
similarity index 69%
rename from plugins/PicoPluginApi0CompatPlugin.php
rename to plugins/PluginApi0Plugin.php
index d459524..9622795 100644
--- a/plugins/PicoPluginApi0CompatPlugin.php
+++ b/plugins/PluginApi0Plugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoPluginApi0CompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PluginApi0Plugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,49 +18,58 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPluginApiPlugin;
+use Pico;
+use PicoDeprecated;
+use ReflectionClass;
+use Twig\Environment as TwigEnvironment;
+
 /**
  * Maintains backward compatibility with plugins using API version 0, written
  * for Pico 0.9 and earlier
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoPluginApi0CompatPlugin extends AbstractPicoPluginApiCompatPlugin
+class PluginApi0Plugin extends AbstractPluginApiPlugin
 {
     /**
-     * This plugin extends {@see PicoPluginApi1CompatPlugin} and
-     * {@see PicoThemeApi0CompatPlugin}
+     * This plugin extends {@see PluginApi1Plugin} and {@see ThemeApi0Plugin}
      *
      * @var string[]
      */
-    protected $dependsOn = array('PicoPluginApi1CompatPlugin', 'PicoThemeApi0CompatPlugin');
+    protected $dependsOn = [ PluginApi1Plugin::class, ThemeApi0Plugin::class ];
 
     /**
      * Map of core events matching event signatures of older API versions
      *
-     * @see AbstractPicoPluginApiCompatPlugin::handleEvent()
+     * @see AbstractPluginApiPlugin::handleEvent()
      *
      * @var array<string,string>
      */
-    protected $eventAliases = array(
-        'onConfigLoaded'      => array('config_loaded'),
-        'onRequestUrl'        => array('request_url'),
-        'onContentLoading'    => array('before_load_content'),
-        'on404ContentLoading' => array('before_404_load_content'),
-        'onMetaParsed'        => array('file_meta'),
-        'onContentParsing'    => array('before_parse_content'),
-        'onContentParsed'     => array('after_parse_content', 'content_parsed'),
-        'onTwigRegistration'  => array('before_twig_register'),
-        'onPageRendered'      => array('after_render')
-    );
+    protected $eventAliases = [
+        'onConfigLoaded' =>      [ 'config_loaded' ],
+        'onRequestUrl' =>        [ 'request_url' ],
+        'onContentLoading' =>    [ 'before_load_content' ],
+        'on404ContentLoading' => [ 'before_404_load_content' ],
+        'onMetaParsed' =>        [ 'file_meta' ],
+        'onContentParsing' =>    [ 'before_parse_content' ],
+        'onContentParsed' =>     [ 'after_parse_content', 'content_parsed' ],
+        'onTwigRegistration' =>  [ 'before_twig_register' ],
+        'onPageRendered' =>      [ 'after_render' ],
+    ];
 
     /**
      * Pico's request file
      *
      * @see Pico::$requestFile
-     * @see PicoPluginApi0CompatPlugin::onRequestFile()
+     * @see PluginApi0Plugin::onRequestFile()
      *
      * @var string|null
      */
@@ -71,7 +80,7 @@ class PicoPluginApi0CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      *
      * @param object[] $plugins loaded plugin instances
      */
-    public function onPluginsLoaded(array &$plugins)
+    public function onPluginsLoaded(array &$plugins): void
     {
         $this->triggerEvent('plugins_loaded');
     }
@@ -86,7 +95,7 @@ public function onPluginsLoaded(array &$plugins)
      *
      * @param array &$config array of config variables
      */
-    public function onConfigLoaded(array &$config)
+    public function onConfigLoaded(array &$config): void
     {
         $this->defineConfigConstants($config);
 
@@ -105,7 +114,7 @@ public function onConfigLoaded(array &$config)
      *
      * @param array &$config array of config variables
      */
-    protected function defineConfigConstants(array &$config)
+    protected function defineConfigConstants(array &$config): void
     {
         if (!defined('ROOT_DIR')) {
             define('ROOT_DIR', $this->getPico()->getRootDir());
@@ -114,7 +123,7 @@ protected function defineConfigConstants(array &$config)
             define('CONFIG_DIR', $this->getPico()->getConfigDir());
         }
         if (!defined('LIB_DIR')) {
-            $picoReflector = new ReflectionClass('Pico');
+            $picoReflector = new ReflectionClass(Pico::class);
             define('LIB_DIR', dirname($picoReflector->getFileName()) . '/');
         }
         if (!defined('PLUGINS_DIR')) {
@@ -136,13 +145,13 @@ protected function defineConfigConstants(array &$config)
     }
 
     /**
-     * Sets PicoPluginApi1CompatPlugin::$requestFile
+     * Sets PluginApi0Plugin::$requestFile
      *
-     * @see PicoPluginApi0CompatPlugin::$requestFile
+     * @see PluginApi0Plugin::$requestFile
      *
      * @param string &$file absolute path to the content file to serve
      */
-    public function onRequestFile(&$file)
+    public function onRequestFile(string &$file): void
     {
         $this->requestFile = &$file;
     }
@@ -152,9 +161,9 @@ public function onRequestFile(&$file)
      *
      * @param string &$rawContent raw file contents
      */
-    public function on404ContentLoaded(&$rawContent)
+    public function on404ContentLoaded(string &$rawContent): void
     {
-        $this->triggerEvent('after_404_load_content', array(&$this->requestFile, &$rawContent));
+        $this->triggerEvent('after_404_load_content', [ &$this->requestFile, &$rawContent ]);
     }
 
     /**
@@ -162,9 +171,9 @@ public function on404ContentLoaded(&$rawContent)
      *
      * @param string &$rawContent raw file contents
      */
-    public function onContentLoaded(&$rawContent)
+    public function onContentLoaded(string &$rawContent): void
     {
-        $this->triggerEvent('after_load_content', array(&$this->requestFile, &$rawContent));
+        $this->triggerEvent('after_load_content', [ &$this->requestFile, &$rawContent ]);
     }
 
     /**
@@ -173,9 +182,9 @@ public function onContentLoaded(&$rawContent)
      * @param string   &$rawContent raw file contents
      * @param string[] &$headers    list of known meta header fields
      */
-    public function onMetaParsing(&$rawContent, array &$headers)
+    public function onMetaParsing(string &$rawContent, array &$headers): void
     {
-        $this->triggerEvent('before_read_file_meta', array(&$headers));
+        $this->triggerEvent('before_read_file_meta', [ &$headers ]);
     }
 
     /**
@@ -183,9 +192,9 @@ public function onMetaParsing(&$rawContent, array &$headers)
      *
      * @param array &$pageData data of the loaded page
      */
-    public function onSinglePageLoaded(array &$pageData)
+    public function onSinglePageLoaded(array &$pageData): void
     {
-        $this->triggerEvent('get_page_data', array(&$pageData, $pageData['meta']));
+        $this->triggerEvent('get_page_data', [ &$pageData, $pageData['meta'] ]);
     }
 
     /**
@@ -207,26 +216,26 @@ public function onPagesLoaded(
         array &$currentPage = null,
         array &$previousPage = null,
         array &$nextPage = null
-    ) {
+    ): void {
         // remove keys of pages array
-        $plainPages = array();
+        $plainPages = [];
         foreach ($pages as &$plainPageData) {
             $plainPages[] = &$plainPageData;
         }
 
         // trigger event
-        $this->triggerEvent('get_pages', array(&$plainPages, &$currentPage, &$previousPage, &$nextPage));
+        $this->triggerEvent('get_pages', [ &$plainPages, &$currentPage, &$previousPage, &$nextPage ]);
 
         // re-index pages array
         $baseUrl = $this->getPico()->getBaseUrl();
         $baseUrlLength = strlen($baseUrl);
         $urlRewritingEnabled = $this->getPico()->isUrlRewritingEnabled();
 
-        $pages = array();
+        $pages = [];
         foreach ($plainPages as &$pageData) {
             if (!isset($pageData['id'])) {
-                if (substr($pageData['url'], 0, $baseUrlLength) === $baseUrl) {
-                    if ($urlRewritingEnabled && (substr($pageData['url'], $baseUrlLength, 1) === '?')) {
+                if (substr_compare($pageData['url'], $baseUrl, 0, $baseUrlLength) === 0) {
+                    if ($urlRewritingEnabled && (substr_compare($pageData['url'], '?', $baseUrlLength, 1) === 0)) {
                         $pageData['id'] = substr($pageData['url'], $baseUrlLength + 1);
                     } else {
                         $pageData['id'] = substr($pageData['url'], $baseUrlLength);
@@ -253,18 +262,18 @@ public function onPagesLoaded(
      * Please note that the `before_render` event gets `$templateName` passed
      * without its file extension. The file extension is re-added later.
      *
-     * @param Twig_Environment &$twig          Twig instance
-     * @param string           &$templateName  file name of the template
-     * @param array            &$twigVariables template variables
+     * @param TwigEnvironment &$twig          Twig instance
+     * @param array           &$twigVariables template variables
+     * @param string          &$templateName  file name of the template
      */
-    public function onPageRendering(Twig_Environment &$twig, array &$twigVariables, &$templateName)
+    public function onPageRendering(TwigEnvironment &$twig, array &$twigVariables, string &$templateName): void
     {
-        $templateNameInfo = pathinfo($templateName) + array('extension' => '');
+        $templateNameInfo = pathinfo($templateName) + [ 'extension' => '' ];
 
         // the template name hasn't had a file extension in API v0
         $templateName = $templateNameInfo['filename'];
 
-        $this->triggerEvent('before_render', array(&$twigVariables, &$twig, &$templateName));
+        $this->triggerEvent('before_render', [ &$twigVariables, &$twig, &$templateName ]);
 
         // recover original file extension
         // we assume that all templates of a theme use the same file extension
@@ -274,7 +283,7 @@ public function onPageRendering(Twig_Environment &$twig, array &$twigVariables,
     /**
      * {@inheritDoc}
      */
-    public function handleCustomEvent($eventName, array $params = array())
+    public function handleCustomEvent(string $eventName, array $params = []): void
     {
         // never trigger custom events
     }
@@ -282,7 +291,7 @@ public function handleCustomEvent($eventName, array $params = array())
     /**
      * {@inheritDoc}
      */
-    public function triggerEvent($eventName, array $params = array())
+    public function triggerEvent(string $eventName, array $params = []): void
     {
         // we don't support compat plugins using API v0, so no need to take care of compat plugins here
         // API v0 events are also triggered on plugins using API v1 (but not later)
@@ -291,7 +300,7 @@ public function triggerEvent($eventName, array $params = array())
 
         foreach ($plugins as $plugin) {
             if (method_exists($plugin, $eventName)) {
-                call_user_func_array(array($plugin, $eventName), $params);
+                call_user_func_array([ $plugin, $eventName ], $params);
             }
         }
     }
@@ -299,7 +308,7 @@ public function triggerEvent($eventName, array $params = array())
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
         return PicoDeprecated::API_VERSION_1;
     }
@@ -307,7 +316,7 @@ public function getApiVersion()
     /**
      * {@inheritDoc}
      */
-    public function getApiVersionSupport()
+    public function getApiVersionSupport(): int
     {
         return PicoDeprecated::API_VERSION_0;
     }
diff --git a/plugins/PicoPluginApi1CompatPlugin.php b/plugins/PluginApi1Plugin.php
similarity index 66%
rename from plugins/PicoPluginApi1CompatPlugin.php
rename to plugins/PluginApi1Plugin.php
index ea886df..0bdab13 100644
--- a/plugins/PicoPluginApi1CompatPlugin.php
+++ b/plugins/PluginApi1Plugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoPluginApi1CompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PluginApi1Plugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,50 +18,58 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPluginApiPlugin;
+use PicoDeprecated;
+use RuntimeException;
+use Twig\Environment as TwigEnvironment;
+
 /**
  * Maintains backward compatibility with plugins using API version 1, written
  * for Pico 1.0
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
+class PluginApi1Plugin extends AbstractPluginApiPlugin
 {
     /**
-     * This plugin extends {@see PicoPluginApi2CompatPlugin} and
-     * {@see PicoThemeApi1CompatPlugin}
+     * This plugin extends {@see PluginApi2Plugin} and {@see ThemeApi1Plugin}
      *
      * @var string[]
      */
-    protected $dependsOn = array('PicoPluginApi2CompatPlugin', 'PicoThemeApi1CompatPlugin');
+    protected $dependsOn = [ PluginApi2Plugin::class, ThemeApi1Plugin::class ];
 
     /**
      * Map of core events matching event signatures of older API versions
      *
-     * @see AbstractPicoPluginApiCompatPlugin::handleEvent()
+     * @see AbstractPluginApiPlugin::handleEvent()
      *
      * @var array<string,string>
      */
-    protected $eventAliases = array(
-        'onConfigLoaded'      => array('onConfigLoaded'),
-        'onRequestUrl'        => array('onRequestUrl'),
-        'onRequestFile'       => array('onRequestFile'),
-        'on404ContentLoaded'  => array('on404ContentLoaded'),
-        'onContentLoaded'     => array('onContentLoaded'),
-        'onContentPrepared'   => array('onContentPrepared'),
-        'onContentParsed'     => array('onContentParsed'),
-        'onPagesLoading'      => array('onPagesLoading'),
-        'onSinglePageLoaded'  => array('onSinglePageLoaded'),
-        'onPageRendered'      => array('onPageRendered')
-    );
+    protected $eventAliases = [
+        'onConfigLoaded' =>     [ 'onConfigLoaded' ],
+        'onRequestUrl' =>       [ 'onRequestUrl' ],
+        'onRequestFile' =>      [ 'onRequestFile' ],
+        'on404ContentLoaded' => [ 'on404ContentLoaded' ],
+        'onContentLoaded' =>    [ 'onContentLoaded' ],
+        'onContentPrepared' =>  [ 'onContentPrepared' ],
+        'onContentParsed' =>    [ 'onContentParsed' ],
+        'onPagesLoading' =>     [ 'onPagesLoading' ],
+        'onSinglePageLoaded' => [ 'onSinglePageLoaded' ],
+        'onPageRendered' =>     [ 'onPageRendered' ],
+    ];
 
     /**
      * Pico's request file
      *
      * @see Pico::$requestFile
-     * @see PicoPluginApi1CompatPlugin::onRequestFile()
+     * @see PluginApi1Plugin::onRequestFile()
      *
      * @var string|null
      */
@@ -71,7 +79,7 @@ class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      * Pico's raw contents
      *
      * @see Pico::$rawContent
-     * @see PicoPluginApi1CompatPlugin::onContentLoaded()
+     * @see PluginApi1Plugin::onContentLoaded()
      *
      * @var string|null
      */
@@ -81,7 +89,7 @@ class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      * Pico's meta headers array
      *
      * @see Pico::$metaHeaders
-     * @see PicoPluginApi1CompatPlugin::onMetaHeaders()
+     * @see PluginApi1Plugin::onMetaHeaders()
      *
      * @var array<string,string>|null
      */
@@ -91,7 +99,7 @@ class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      * Pico's pages array
      *
      * @see Pico::$pages
-     * @see PicoPluginApi1CompatPlugin::onPagesLoaded()
+     * @see PluginApi1Plugin::onPagesLoaded()
      *
      * @var array[]|null
      */
@@ -101,9 +109,9 @@ class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      * Pico's Twig instance
      *
      * @see Pico::$twig
-     * @see PicoPluginApi1CompatPlugin::onTwigRegistered()
+     * @see PluginApi1Plugin::onTwigRegistered()
      *
-     * @var Twig_Environment|null
+     * @var TwigEnvironment|null
      */
     protected $twig;
 
@@ -118,11 +126,11 @@ class PicoPluginApi1CompatPlugin extends AbstractPicoPluginApiCompatPlugin
      *
      * @param object[] $plugins loaded plugin instances
      */
-    public function onPluginsLoaded(array $plugins)
+    public function onPluginsLoaded(array $plugins): void
     {
         $originalPlugins = $plugins;
 
-        $this->triggerEvent('onPluginsLoaded', array(&$plugins));
+        $this->triggerEvent('onPluginsLoaded', [ &$plugins ]);
 
         foreach ($plugins as $pluginName => $plugin) {
             if (!isset($originalPlugins[$pluginName])) {
@@ -147,13 +155,13 @@ public function onPluginsLoaded(array $plugins)
     }
 
     /**
-     * Sets PicoPluginApi1CompatPlugin::$requestFile
+     * Sets PluginApi1Plugin::$requestFile
      *
-     * @see PicoPluginApi1CompatPlugin::$requestFile
+     * @see PluginApi1Plugin::$requestFile
      *
      * @param string &$file absolute path to the content file to serve
      */
-    public function onRequestFile(&$file)
+    public function onRequestFile(string &$file): void
     {
         $this->requestFile = &$file;
     }
@@ -161,19 +169,19 @@ public function onRequestFile(&$file)
     /**
      * Triggers the onContentLoading event
      */
-    public function onContentLoading()
+    public function onContentLoading(): void
     {
-        $this->triggerEvent('onContentLoading', array(&$this->requestFile));
+        $this->triggerEvent('onContentLoading', [ &$this->requestFile ]);
     }
 
     /**
-     * Sets PicoPluginApi1CompatPlugin::$rawContent
+     * Sets PluginApi1Plugin::$rawContent
      *
-     * @see PicoPluginApi1CompatPlugin::$rawContent
+     * @see PluginApi1Plugin::$rawContent
      *
      * @param string &$rawContent raw file contents
      */
-    public function onContentLoaded(&$rawContent)
+    public function onContentLoaded(string &$rawContent): void
     {
         $this->rawContent = &$rawContent;
     }
@@ -181,20 +189,20 @@ public function onContentLoaded(&$rawContent)
     /**
      * Triggers the on404ContentLoading event
      */
-    public function on404ContentLoading()
+    public function on404ContentLoading(): void
     {
-        $this->triggerEvent('on404ContentLoading', array(&$this->requestFile));
+        $this->triggerEvent('on404ContentLoading', [ &$this->requestFile ]);
     }
 
     /**
      * Triggers the onMetaParsing event
      *
-     * @see PicoPluginApi1CompatPlugin::onMetaHeaders()
+     * @see PluginApi1Plugin::onMetaHeaders()
      */
-    public function onMetaParsing()
+    public function onMetaParsing(): void
     {
         $headersFlipped = $this->getFlippedMetaHeaders();
-        $this->triggerEvent('onMetaParsing', array(&$this->rawContent, &$headersFlipped));
+        $this->triggerEvent('onMetaParsing', [ &$this->rawContent, &$headersFlipped ]);
         $this->updateFlippedMetaHeaders($headersFlipped);
     }
 
@@ -203,28 +211,28 @@ public function onMetaParsing()
      *
      * @param string[] &$meta parsed meta data
      */
-    public function onMetaParsed(array &$meta)
+    public function onMetaParsed(array &$meta): void
     {
-        $this->triggerEvent('onMetaParsed', array(&$meta));
+        $this->triggerEvent('onMetaParsed', [ &$meta ]);
         $this->triggerEvent('onParsedownRegistration');
     }
 
     /**
      * Triggers the onContentParsing event
      */
-    public function onContentParsing()
+    public function onContentParsing(): void
     {
-        $this->triggerEvent('onContentParsing', array(&$this->rawContent));
+        $this->triggerEvent('onContentParsing', [ &$this->rawContent ]);
     }
 
     /**
-     * Sets PicoPluginApi1CompatPlugin::$pages
+     * Sets PluginApi1Plugin::$pages
      *
-     * @see PicoPluginApi1CompatPlugin::$pages
+     * @see PluginApi1Plugin::$pages
      *
      * @param array[] &$pages sorted list of all known pages
      */
-    public function onPagesLoaded(array &$pages)
+    public function onPagesLoaded(array &$pages): void
     {
         $this->pages = &$pages;
     }
@@ -240,8 +248,8 @@ public function onCurrentPageDiscovered(
         array &$currentPage = null,
         array &$previousPage = null,
         array &$nextPage = null
-    ) {
-        $this->triggerEvent('onPagesLoaded', array(&$this->pages, &$currentPage, &$previousPage, &$nextPage));
+    ): void {
+        $this->triggerEvent('onPagesLoaded', [ &$this->pages, &$currentPage, &$previousPage, &$nextPage ]);
 
         $this->triggerEvent('onTwigRegistration');
         $this->getPico()->getTwig();
@@ -253,38 +261,38 @@ public function onCurrentPageDiscovered(
      * @param string &$templateName  file name of the template
      * @param array  &$twigVariables template variables
      */
-    public function onPageRendering(&$templateName, array &$twigVariables)
+    public function onPageRendering(string &$templateName, array &$twigVariables): void
     {
-        $this->triggerEvent('onPageRendering', array(&$this->twig, &$twigVariables, &$templateName));
+        $this->triggerEvent('onPageRendering', [ &$this->twig, &$twigVariables, &$templateName ]);
     }
 
     /**
      * Triggers the onMetaHeaders event with flipped meta headers and sets
-     * PicoPluginApi1CompatPlugin::$metaHeaders
+     * PluginApi1Plugin::$metaHeaders
      *
-     * @see PicoPluginApi1CompatPlugin::$metaHeaders
+     * @see PluginApi1Plugin::$metaHeaders
      *
      * @param string[] &$headers list of known meta header fields; the array
      *     key specifies the YAML key to search for, the array value is later
      *     used to access the found value
      */
-    public function onMetaHeaders(array &$headers)
+    public function onMetaHeaders(array &$headers): void
     {
         $this->metaHeaders = &$headers;
 
         $headersFlipped = $this->getFlippedMetaHeaders();
-        $this->triggerEvent('onMetaHeaders', array(&$headersFlipped));
+        $this->triggerEvent('onMetaHeaders', [ &$headersFlipped ]);
         $this->updateFlippedMetaHeaders($headersFlipped);
     }
 
     /**
-     * Sets PicoPluginApi1CompatPlugin::$twig
+     * Sets PluginApi1Plugin::$twig
      *
-     * @see PicoPluginApi1CompatPlugin::$twig
+     * @see PluginApi1Plugin::$twig
      *
-     * @param Twig_Environment &$twig Twig instance
+     * @param TwigEnvironment &$twig Twig instance
      */
-    public function onTwigRegistered(Twig_Environment &$twig)
+    public function onTwigRegistered(TwigEnvironment &$twig): void
     {
         $this->twig = $twig;
     }
@@ -301,22 +309,22 @@ public function onTwigRegistered(Twig_Environment &$twig)
      *
      * @return array flipped meta headers
      */
-    protected function getFlippedMetaHeaders()
+    protected function getFlippedMetaHeaders(): array
     {
         if ($this->metaHeaders === null) {
             // make sure to trigger the onMetaHeaders event
             $this->getPico()->getMetaHeaders();
         }
 
-        return array_flip($this->metaHeaders ?: array());
+        return array_flip($this->metaHeaders ?: []);
     }
 
     /**
-     * Syncs PicoPluginApi1CompatPlugin::$metaHeaders with a flipped headers array
+     * Syncs PluginApi1Plugin::$metaHeaders with a flipped headers array
      *
      * @param array $headersFlipped flipped headers array
      */
-    protected function updateFlippedMetaHeaders(array $headersFlipped)
+    protected function updateFlippedMetaHeaders(array $headersFlipped): void
     {
         foreach ($this->metaHeaders as $name => $key) {
             if (!isset($headersFlipped[$key])) {
@@ -332,7 +340,7 @@ protected function updateFlippedMetaHeaders(array $headersFlipped)
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
         return PicoDeprecated::API_VERSION_2;
     }
@@ -340,7 +348,7 @@ public function getApiVersion()
     /**
      * {@inheritDoc}
      */
-    public function getApiVersionSupport()
+    public function getApiVersionSupport(): int
     {
         return PicoDeprecated::API_VERSION_1;
     }
diff --git a/plugins/PluginApi2Plugin.php b/plugins/PluginApi2Plugin.php
new file mode 100644
index 0000000..84ba0b7
--- /dev/null
+++ b/plugins/PluginApi2Plugin.php
@@ -0,0 +1,137 @@
+<?php
+/**
+ * This file is part of Pico. It's copyrighted by the contributors recorded
+ * in the version control history of the file, available from the following
+ * original location:
+ *
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PluginApi2Plugin.php>
+ *
+ * This file was created by splitting up an original file into multiple files,
+ * which in turn was previously part of the project's main repository. The
+ * version control history of these files apply accordingly, available from
+ * the following original locations:
+ *
+ * <https://github.com/picocms/pico-deprecated/blob/90ea3d5a9767f1511f165e051dd7ffb8f1b3f92e/PicoDeprecated.php>
+ * <https://github.com/picocms/Pico/blob/82a342ba445122182b898a2c1800f03c8d16f18c/plugins/00-PicoDeprecated.php>
+ *
+ * SPDX-License-Identifier: MIT
+ * License-Filename: LICENSE
+ */
+
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPluginApiPlugin;
+use PicoDeprecated;
+
+/**
+ * Maintains backward compatibility with plugins using API version 2, written
+ * for Pico 2.0
+ *
+ * @author  Daniel Rudolf
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
+ */
+class PluginApi2Plugin extends AbstractPluginApiPlugin
+{
+    /**
+     * This plugin extends {@see ThemeApi2Plugin}
+     *
+     * @var string[]
+     */
+    protected $dependsOn = [ PluginApi3Plugin::class, ThemeApi2Plugin::class ];
+
+    /**
+     * Map of core events matching event signatures of older API versions
+     *
+     * @see AbstractPluginApiPlugin::handleEvent()
+     *
+     * @var array<string,string>
+     */
+    protected $eventAliases = [
+        'onPluginsLoaded' =>         [ 'onPluginsLoaded' ],
+        'onPluginManuallyLoaded' =>  [ 'onPluginManuallyLoaded' ],
+        'onRequestUrl' =>            [ 'onRequestUrl' ],
+        'onRequestFile' =>           [ 'onRequestFile' ],
+        'onContentLoading' =>        [ 'onContentLoading' ],
+        'on404ContentLoading' =>     [ 'on404ContentLoading' ],
+        'on404ContentLoaded' =>      [ 'on404ContentLoaded' ],
+        'onContentLoaded' =>         [ 'onContentLoaded' ],
+        'onMetaParsing' =>           [ 'onMetaParsing' ],
+        'onMetaParsed' =>            [ 'onMetaParsed' ],
+        'onContentParsing' =>        [ 'onContentParsing' ],
+        'onContentPrepared' =>       [ 'onContentPrepared' ],
+        'onContentParsed' =>         [ 'onContentParsed' ],
+        'onPagesLoading' =>          [ 'onPagesLoading' ],
+        'onSinglePageLoading' =>     [ 'onSinglePageLoading' ],
+        'onSinglePageContent' =>     [ 'onSinglePageContent' ],
+        'onSinglePageLoaded' =>      [ 'onSinglePageLoaded' ],
+        'onPagesDiscovered' =>       [ 'onPagesDiscovered' ],
+        'onPagesLoaded' =>           [ 'onPagesLoaded' ],
+        'onCurrentPageDiscovered' => [ 'onCurrentPageDiscovered' ],
+        'onPageTreeBuilt' =>         [ 'onPageTreeBuilt' ],
+        'onPageRendering' =>         [ 'onPageRendering' ],
+        'onPageRendered' =>          [ 'onPageRendered' ],
+        'onMetaHeaders' =>           [ 'onMetaHeaders' ],
+        'onYamlParserRegistered' =>  [ 'onYamlParserRegistered' ],
+        'onParsedownRegistered' =>   [ 'onParsedownRegistered' ],
+        'onTwigRegistered' =>        [ 'onTwigRegistered' ],
+    ];
+
+    /**
+     * Pico's config array
+     *
+     * @see Pico::$config
+     * @see PluginApi2Plugin::onConfigLoaded()
+     *
+     * @var array|null
+     */
+    protected $config;
+
+    /**
+     * Sets PluginApi2Plugin::$config and handles the theme_url config param
+     *
+     * @see PluginApi2Plugin::$config
+     *
+     * @param array $config
+     */
+    public function onConfigLoaded(array &$config): void
+    {
+        $this->config = &$config;
+
+        if (!empty($config['theme_url'])) {
+            $config['themes_url'] = $this->getPico()->getAbsoluteUrl($config['theme_url']);
+            $config['theme_url'] = &$config['themes_url'];
+        }
+    }
+
+    /**
+     * Triggers the onConfigLoaded event
+     *
+     * @param string $theme           name of current theme
+     * @param int    $themeApiVersion API version of the theme
+     * @param array  &$themeConfig    config array of the theme
+     */
+    public function onThemeLoaded(string $theme, int $themeApiVersion, array &$themeConfig): void
+    {
+        $this->triggerEvent('onConfigLoaded', [ &$this->config ]);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function getApiVersion(): int
+    {
+        return PicoDeprecated::API_VERSION_3;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function getApiVersionSupport(): int
+    {
+        return PicoDeprecated::API_VERSION_2;
+    }
+}
diff --git a/plugins/PluginApi3Plugin.php b/plugins/PluginApi3Plugin.php
new file mode 100644
index 0000000..df5a3e5
--- /dev/null
+++ b/plugins/PluginApi3Plugin.php
@@ -0,0 +1,86 @@
+<?php
+/**
+ * This file is part of Pico. It's copyrighted by the contributors recorded
+ * in the version control history of the file, available from the following
+ * original location:
+ *
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PluginApi3Plugin.php>
+ *
+ * SPDX-License-Identifier: MIT
+ * License-Filename: LICENSE
+ */
+
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPluginApiPlugin;
+use PicoDeprecated;
+
+/**
+ * Maintains backward compatibility with plugins using API version 3, written
+ * for Pico 2.1
+ *
+ * @author  Daniel Rudolf
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
+ */
+class PluginApi3Plugin extends AbstractPluginApiPlugin
+{
+    /**
+     * Map of core events matching event signatures of older API versions
+     *
+     * @see AbstractPluginApiPlugin::handleEvent()
+     *
+     * @var array<string,string>
+     */
+    protected $eventAliases = [
+        'onPluginsLoaded' =>         [ 'onPluginsLoaded' ],
+        'onPluginManuallyLoaded' =>  [ 'onPluginManuallyLoaded' ],
+        'onConfigLoaded' =>          [ 'onConfigLoaded' ],
+        'onThemeLoading' =>          [ 'onThemeLoading' ],
+        'onThemeLoaded' =>           [ 'onThemeLoaded' ],
+        'onRequestUrl' =>            [ 'onRequestUrl' ],
+        'onRequestFile' =>           [ 'onRequestFile' ],
+        'onContentLoading' =>        [ 'onContentLoading' ],
+        'on404ContentLoading' =>     [ 'on404ContentLoading' ],
+        'on404ContentLoaded' =>      [ 'on404ContentLoaded' ],
+        'onContentLoaded' =>         [ 'onContentLoaded' ],
+        'onMetaParsing' =>           [ 'onMetaParsing' ],
+        'onMetaParsed' =>            [ 'onMetaParsed' ],
+        'onContentParsing' =>        [ 'onContentParsing' ],
+        'onContentPrepared' =>       [ 'onContentPrepared' ],
+        'onContentParsed' =>         [ 'onContentParsed' ],
+        'onPagesLoading' =>          [ 'onPagesLoading' ],
+        'onSinglePageLoading' =>     [ 'onSinglePageLoading' ],
+        'onSinglePageContent' =>     [ 'onSinglePageContent' ],
+        'onSinglePageLoaded' =>      [ 'onSinglePageLoaded' ],
+        'onPagesDiscovered' =>       [ 'onPagesDiscovered' ],
+        'onPagesLoaded' =>           [ 'onPagesLoaded' ],
+        'onCurrentPageDiscovered' => [ 'onCurrentPageDiscovered' ],
+        'onPageTreeBuilt' =>         [ 'onPageTreeBuilt' ],
+        'onPageRendering' =>         [ 'onPageRendering' ],
+        'onPageRendered' =>          [ 'onPageRendered' ],
+        'onMetaHeaders' =>           [ 'onMetaHeaders' ],
+        'onYamlParserRegistered' =>  [ 'onYamlParserRegistered' ],
+        'onParsedownRegistered' =>   [ 'onParsedownRegistered' ],
+        'onTwigRegistered' =>        [ 'onTwigRegistered' ],
+    ];
+
+    /**
+     * {@inheritDoc}
+     */
+    public function getApiVersion(): int
+    {
+        return PicoDeprecated::API_VERSION_4;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function getApiVersionSupport(): int
+    {
+        return PicoDeprecated::API_VERSION_3;
+    }
+}
diff --git a/plugins/PicoThemeApi0CompatPlugin.php b/plugins/ThemeApi0Plugin.php
similarity index 67%
rename from plugins/PicoThemeApi0CompatPlugin.php
rename to plugins/ThemeApi0Plugin.php
index 9ed6a64..b378c17 100644
--- a/plugins/PicoThemeApi0CompatPlugin.php
+++ b/plugins/ThemeApi0Plugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoThemeApi0CompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/ThemeApi0Plugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,34 +18,41 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPlugin;
+use PicoDeprecated;
+
 /**
  * Maintains backward compatibility with themes using API version 0, written
  * for Pico 0.9 and earlier
  *
  * Since there were no theme-related changes between Pico 0.9 and Pico 1.0,
  * this compat plugin doesn't hold any code itself, it just depends on
- * {@see PicoThemeApi1CompatPlugin}. Since themes didn't support API versioning
- * until Pico 2.1 (i.e. API version 3), all older themes will appear to use API
+ * {@see ThemeApi1Plugin}. Since themes didn't support API versioning until
+ * Pico 2.1 (i.e. API version 3), all older themes will appear to use API
  * version 0.
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoThemeApi0CompatPlugin extends AbstractPicoCompatPlugin
+class ThemeApi0Plugin extends AbstractPlugin
 {
     /**
-     * This plugin extends {@see PicoThemeApi1CompatPlugin}
+     * This plugin extends {@see ThemeApi1Plugin}
      *
      * @var string[]
      */
-    protected $dependsOn = array('PicoThemeApi1CompatPlugin');
+    protected $dependsOn = [ ThemeApi1Plugin::class ];
 
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
         return PicoDeprecated::API_VERSION_3;
     }
diff --git a/plugins/PicoThemeApi1CompatPlugin.php b/plugins/ThemeApi1Plugin.php
similarity index 78%
rename from plugins/PicoThemeApi1CompatPlugin.php
rename to plugins/ThemeApi1Plugin.php
index f4b276b..ee17208 100644
--- a/plugins/PicoThemeApi1CompatPlugin.php
+++ b/plugins/ThemeApi1Plugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoThemeApi1CompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/ThemeApi1Plugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,32 +18,40 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPlugin;
+use PicoDeprecated;
+use Twig\Error\LoaderError as TwigLoaderError;
+
 /**
  * Maintains backward compatibility with themes using API version 1, written
  * for Pico 1.0
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoThemeApi1CompatPlugin extends AbstractPicoCompatPlugin
+class ThemeApi1Plugin extends AbstractPlugin
 {
     /**
-     * This plugin extends {@see PicoThemeApi2CompatPlugin}
+     * This plugin extends {@see ThemeApi2Plugin}
      *
      * @var string[]
      */
-    protected $dependsOn = array('PicoThemeApi2CompatPlugin');
+    protected $dependsOn = [ ThemeApi2Plugin::class ];
 
     /**
      * Lowers the page's meta headers
      *
-     * @see PicoThemeApi1CompatPlugin::lowerFileMeta()
+     * @see ThemeApi1Plugin::lowerFileMeta()
      *
      * @param string[] &$meta parsed meta data
      */
-    public function onMetaParsed(array &$meta)
+    public function onMetaParsed(array &$meta): void
     {
         $this->lowerFileMeta($meta);
     }
@@ -51,11 +59,11 @@ public function onMetaParsed(array &$meta)
     /**
      * Lowers the page's meta headers
      *
-     * @see PicoThemeApi1CompatPlugin::lowerFileMeta()
+     * @see ThemeApi1Plugin::lowerFileMeta()
      *
      * @param array &$pageData data of the loaded page
      */
-    public function onSinglePageLoaded(array &$pageData)
+    public function onSinglePageLoaded(array &$pageData): void
     {
         // don't lower the file meta of the requested page,
         // it was already lowered during the onMetaParsed event
@@ -67,13 +75,13 @@ public function onSinglePageLoaded(array &$pageData)
     }
 
     /**
-     * Handles .html Twig templates and re-introcudes the Twig variables
+     * Handles .html Twig templates and re-introduces the Twig variables
      * rewrite_url and is_front_page
      *
      * @param string &$templateName  file name of the template
      * @param array  &$twigVariables template variables
      */
-    public function onPageRendering(&$templateName, array &$twigVariables)
+    public function onPageRendering(string &$templateName, array &$twigVariables): void
     {
         if (!isset($twigVariables['rewrite_url'])) {
             $twigVariables['rewrite_url'] = $this->getPico()->isUrlRewritingEnabled();
@@ -88,19 +96,18 @@ public function onPageRendering(&$templateName, array &$twigVariables)
 
         // API v2 requires themes to use .twig as file extension
         // try to load the template and if this fails, try .html instead (as of API v1)
-        $templateNameInfo = pathinfo($templateName) + array('extension' => '');
+        $templateNameInfo = pathinfo($templateName) + [ 'extension' => '' ];
         $twig = $this->getPico()->getTwig();
 
         try {
-            $twig->loadTemplate($templateName);
-        } catch (Twig_Error_Loader $e) {
+            $twig->load($templateName);
+        } catch (TwigLoaderError $e) {
             if ($templateNameInfo['extension'] === 'twig') {
                 try {
-                    $twig->loadTemplate($templateNameInfo['filename'] . '.html');
+                    $twig->load($templateNameInfo['filename'] . '.html');
 
                     $templateName = $templateNameInfo['filename'] . '.html';
-                    $templateNameInfo['extension'] = 'html';
-                } catch (Twig_Error_Loader $e) {
+                } catch (TwigLoaderError $e) {
                     // template doesn't exist, Twig will very likely fail later
                 }
             }
@@ -115,12 +122,12 @@ public function onPageRendering(&$templateName, array &$twigVariables)
      *
      * @param array &$meta meta data
      */
-    protected function lowerFileMeta(array &$meta)
+    protected function lowerFileMeta(array &$meta): void
     {
         $metaHeaders = $this->getPico()->getMetaHeaders();
 
         // get unregistered meta
-        $unregisteredMeta = array();
+        $unregisteredMeta = [];
         foreach ($meta as $key => $value) {
             if (!in_array($key, $metaHeaders)) {
                 $unregisteredMeta[$key] = &$meta[$key];
@@ -147,7 +154,7 @@ protected function lowerFileMeta(array &$meta)
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
         return PicoDeprecated::API_VERSION_3;
     }
diff --git a/plugins/PicoThemeApi2CompatPlugin.php b/plugins/ThemeApi2Plugin.php
similarity index 71%
rename from plugins/PicoThemeApi2CompatPlugin.php
rename to plugins/ThemeApi2Plugin.php
index 646925b..73de6c8 100644
--- a/plugins/PicoThemeApi2CompatPlugin.php
+++ b/plugins/ThemeApi2Plugin.php
@@ -4,7 +4,7 @@
  * in the version control history of the file, available from the following
  * original location:
  *
- * <https://github.com/picocms/pico-deprecated/blob/master/plugins/PicoThemeApi2CompatPlugin.php>
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/ThemeApi2Plugin.php>
  *
  * This file was created by splitting up an original file into multiple files,
  * which in turn was previously part of the project's main repository. The
@@ -18,17 +18,36 @@
  * License-Filename: LICENSE
  */
 
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPlugin;
+use PicoDeprecated;
+use ReflectionObject;
+use Twig\Environment as TwigEnvironment;
+use Twig\Error\LoaderError as TwigLoaderError;
+use Twig\Extension\EscaperExtension as TwigEscaperExtension;
+use Twig\Loader\LoaderInterface as TwigLoaderInterface;
+
 /**
  * Maintains backward compatibility with themes using API version 2, written
  * for Pico 2.0
  *
  * @author  Daniel Rudolf
- * @link    http://picocms.org
- * @license http://opensource.org/licenses/MIT The MIT License
- * @version 2.1
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
  */
-class PicoThemeApi2CompatPlugin extends AbstractPicoCompatPlugin
+class ThemeApi2Plugin extends AbstractPlugin
 {
+    /**
+     * This plugin extends {@see ThemeApi3Plugin}
+     *
+     * @var string[]
+     */
+    protected $dependsOn = [ ThemeApi3Plugin::class ];
+
     /**
      * Manually configured Twig escape strategy
      *
@@ -41,16 +60,16 @@ class PicoThemeApi2CompatPlugin extends AbstractPicoCompatPlugin
      *
      * @var string[]
      */
-    protected $pluginPaths = array();
+    protected $pluginPaths = [];
 
     /**
-     * Sets PicoThemeApi2CompatPlugin::$twigEscapeStrategy
+     * Sets ThemeApi2Plugin::$twigEscapeStrategy
      *
-     * @see PicoThemeApi2CompatPlugin::$twigEscapeStrategy
+     * @see ThemeApi2Plugin::$twigEscapeStrategy
      *
      * @param array &$config array of config variables
      */
-    public function onConfigLoaded(array &$config)
+    public function onConfigLoaded(array &$config): void
     {
         if (isset($config['twig_config']['autoescape'])) {
             $this->twigEscapeStrategy = $config['twig_config']['autoescape'];
@@ -63,7 +82,7 @@ public function onConfigLoaded(array &$config)
      * @param string &$templateName  file name of the template
      * @param array  &$twigVariables template variables
      */
-    public function onPageRendering(&$templateName, array &$twigVariables)
+    public function onPageRendering(string &$templateName, array &$twigVariables): void
     {
         $twigVariables['prev_page'] = &$twigVariables['previous_page'];
         $twigVariables['base_dir'] = rtrim($this->getPico()->getRootDir(), '/');
@@ -71,19 +90,19 @@ public function onPageRendering(&$templateName, array &$twigVariables)
     }
 
     /**
-     * Registers PicoPluginApi2CompatPlugin::twigEscapeStrategy() as Twig's
-     * default escape strategy
+     * Registers ThemeApi2Plugin::twigEscapeStrategy() as Twig's default
+     * escape strategy
      *
-     * @see PicoPluginApi2CompatPlugin::twigEscapeStrategy()
+     * @see ThemeApi2Plugin::twigEscapeStrategy()
      *
-     * @param Twig_Environment &$twig Twig instance
+     * @param TwigEnvironment &$twig Twig instance
      */
-    public function onTwigRegistered(Twig_Environment &$twig)
+    public function onTwigRegistered(TwigEnvironment &$twig): void
     {
-        if ($twig->hasExtension('Twig_Extension_Escaper')) {
-            /** @var Twig_Extension_Escaper $escaperExtension */
-            $escaperExtension = $twig->getExtension('Twig_Extension_Escaper');
-            $escaperExtension->setDefaultStrategy(array($this, 'twigEscapeStrategy'));
+        if ($twig->hasExtension(TwigEscaperExtension::class)) {
+            /** @var TwigEscaperExtension $escaperExtension */
+            $escaperExtension = $twig->getExtension(TwigEscaperExtension::class);
+            $escaperExtension->setDefaultStrategy([ $this, 'twigEscapeStrategy' ]);
         }
     }
 
@@ -101,7 +120,7 @@ public function onTwigRegistered(Twig_Environment &$twig)
      *
      * @return string|false escape strategy for this template
      */
-    public function twigEscapeStrategy($templateName)
+    public function twigEscapeStrategy(string $templateName)
     {
         $twigConfig = $this->getPico()->getConfig('twig_config');
         $escapeStrategy = $twigConfig['autoescape'];
@@ -118,24 +137,18 @@ public function twigEscapeStrategy($templateName)
             return false;
         }
 
-        /** @var Twig_SourceContextLoaderInterface $twigLoader */
+        /** @var TwigLoaderInterface $twigLoader */
         $twigLoader = $this->getPico()->getTwig()->getLoader();
-        if (!$twigLoader instanceof Twig_SourceContextLoaderInterface) {
-            throw new RuntimeException(
-                "PicoDeprecated compat plugin '" . __CLASS__ . "' requires a 'Twig_SourceContextLoaderInterface' "
-                . "Twig loader, '" . get_class($twigLoader) . "' given"
-            );
-        }
 
         try {
             $templatePath = $twigLoader->getSourceContext($templateName)->getPath();
-        } catch (\Twig\Error\LoaderError $e) {
+        } catch (TwigLoaderError $e) {
             $templatePath = '';
         }
 
         if ($templatePath) {
             $themePath = realpath($this->getPico()->getThemesDir() . $this->getPico()->getTheme()) . '/';
-            if (substr($templatePath, 0, strlen($themePath)) === $themePath) {
+            if (substr_compare($templatePath, $themePath, 0, strlen($themePath)) === 0) {
                 $themeApiVersion = $this->getPico()->getThemeApiVersion();
                 return ($themeApiVersion >= PicoDeprecated::API_VERSION_3) ? $escapeStrategy : false;
             }
@@ -160,11 +173,11 @@ public function twigEscapeStrategy($templateName)
      *
      * @return object|null either the matching plugin instance or NULL
      */
-    protected function getPluginFromPath($path)
+    protected function getPluginFromPath(string $path): ?object
     {
         $plugins = $this->getPico()->getPlugins();
         foreach ($this->pluginPaths as $pluginName => $pluginPath) {
-            if ($pluginPath && (substr($path, 0, strlen($pluginPath)) === $pluginPath)) {
+            if ($pluginPath && (substr_compare($path, $pluginPath, 0, strlen($pluginPath)) === 0)) {
                 return $plugins[$pluginName];
             }
         }
@@ -181,13 +194,13 @@ protected function getPluginFromPath($path)
             $pluginReflector = new ReflectionObject($plugin);
 
             $pluginPath = dirname($pluginReflector->getFileName() ?: '') . '/';
-            if (in_array($pluginPath, array('/', $rootDir, $vendorDir, $pluginsDir, $themesDir), true)) {
+            if (in_array($pluginPath, [ '/', $rootDir, $vendorDir, $pluginsDir, $themesDir ], true)) {
                 $pluginPath = '';
             }
 
             $this->pluginPaths[$pluginName] = $pluginPath;
 
-            if ($pluginPath && (substr($path, 0, strlen($pluginPath)) === $pluginPath)) {
+            if ($pluginPath && (substr_compare($path, $pluginPath, 0, strlen($pluginPath)) === 0)) {
                 return $plugins[$pluginName];
             }
         }
@@ -198,7 +211,7 @@ protected function getPluginFromPath($path)
     /**
      * {@inheritDoc}
      */
-    public function getApiVersion()
+    public function getApiVersion(): int
     {
         return PicoDeprecated::API_VERSION_3;
     }
diff --git a/plugins/ThemeApi3Plugin.php b/plugins/ThemeApi3Plugin.php
new file mode 100644
index 0000000..8a84259
--- /dev/null
+++ b/plugins/ThemeApi3Plugin.php
@@ -0,0 +1,38 @@
+<?php
+/**
+ * This file is part of Pico. It's copyrighted by the contributors recorded
+ * in the version control history of the file, available from the following
+ * original location:
+ *
+ * <https://github.com/picocms/pico-deprecated/blob/master/plugins/ThemeApi3Plugin.php>
+ *
+ * SPDX-License-Identifier: MIT
+ * License-Filename: LICENSE
+ */
+
+declare(strict_types=1);
+
+namespace picocms\PicoDeprecated\Plugin;
+
+use picocms\PicoDeprecated\AbstractPlugin;
+use PicoDeprecated;
+
+/**
+ * Maintains backward compatibility with themes using API version 3, written
+ * for Pico 2.1
+ *
+ * @author  Daniel Rudolf
+ * @link    https://picocms.org
+ * @license https://opensource.org/licenses/MIT The MIT License
+ * @version 3.0
+ */
+class ThemeApi3Plugin extends AbstractPlugin
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function getApiVersion(): int
+    {
+        return PicoDeprecated::API_VERSION_4;
+    }
+}