diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index f343710ca6..5d6fdc1440 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -129,21 +129,26 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        php: [ '8.0', '7.4', '7.3', '7.2', '7.1', '7.0', '5.6.20' ]
+        php: [ '5.6' ]
+        phpunit: [ '4-php-5.6' ]
         os: [ ubuntu-latest ]
         memcached: [ false ]
         include:
-          # Include job for PHP 7.4 with memcached.
-          - php: '7.4'
-            os: ubuntu-latest
-            memcached: true
-          # Report the results of the PHP 7.4 without memcached job.
-          - php: '7.4'
+          - php: '5.5'
+            phpunit: '5.5'
+            os: ubuntu-latest
+            memcached: false
+          - php: '5.4'
+            phpunit: '5.4'
+            os: ubuntu-latest
+            memcached: false
+          - php: '5.3'
+            phpunit: '5.3'
             os: ubuntu-latest
             memcached: false
-            report: true
     env:
       LOCAL_PHP: ${{ matrix.php }}-fpm
+      LOCAL_PHPUNIT: ${{ matrix.phpunit }}-fpm
       LOCAL_PHP_MEMCACHED: ${{ matrix.memcached }}
 
     steps:
@@ -179,13 +184,8 @@ jobs:
       - name: Install Dependencies
         run: npx install-changed --install-command="npm ci"
 
-      - name: Get composer cache directory
-        id: composer-cache
-        if: ${{ env.COMPOSER_INSTALL == true || env.LOCAL_PHP == '8.0-fpm' }}
-        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
-
       - name: Cache Composer dependencies
-        if: ${{ env.COMPOSER_INSTALL == true || env.LOCAL_PHP == '8.0-fpm' }}
+        if: ${{ env.COMPOSER_INSTALL == true }}
         uses: actions/cache@v2
         env:
           cache-name: cache-composer-dependencies
@@ -196,19 +196,10 @@ jobs:
             ${{ runner.os }}-php-${{ matrix.php }}-composer-
 
       - name: Install Composer dependencies
-        if: ${{ env.COMPOSER_INSTALL == true || env.LOCAL_PHP == '8.0-fpm' }}
+        if: ${{ env.COMPOSER_INSTALL == true }}
         run: |
           docker-compose run --rm php composer --version
-
-          # The PHPUnit 7.x phar is not compatible with PHP 8 and won't be updated,
-          # as PHPUnit 7 is no longer supported. The Composer-installed PHPUnit should be
-          # used for PHP 8 testing instead.
-          if [ ${{ env.LOCAL_PHP }} == '8.0-fpm' ]; then
-            docker-compose run --rm php composer install --ignore-platform-reqs
-            echo "PHPUNIT_SCRIPT=php-composer" >> $GITHUB_ENV
-          else
-            docker-compose run --rm php composer install
-          fi
+          docker-compose run --rm php composer install
 
       - name: Docker debug information
         run: |
@@ -266,11 +257,8 @@ jobs:
       - name: Run REST API tests
         run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c phpunit.xml.dist --group restapi-jsclient
 
-      # Xdebug supports PHP 8 only from version 3.0, which is not released yet.
-      # Once Xdebug 3.0 is released and included in the Docker image, the IF condition should be removed.
       # __fakegroup__ is excluded to force PHPUnit to ignore the <exclude> settings in phpunit.xml.dist.
       - name: Run (xDebug) tests
-        if: ${{ env.LOCAL_PHP != '8.0-fpm' }}
         run: LOCAL_PHP_XDEBUG=true npm run test:php -- -v --group xdebug --exclude-group __fakegroup__
 
       - name: Checkout the WordPress Test Reporter