diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index aba74879f0..b368524ca5 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -30,9 +30,6 @@ concurrency:
 
 env:
   PUPPETEER_SKIP_CHROMIUM_DOWNLOAD: ${{ true }}
-  COMPOSER_INSTALL: ${{ false }}
-  # Controls which NPM script to use for running PHPUnit tests. Options ar `php` and `php-composer`.
-  PHPUNIT_SCRIPT: php
   LOCAL_PHP_MEMCACHED: ${{ false }}
   SLOW_TESTS: 'external-http,media,restapi'
 
@@ -121,33 +118,34 @@ jobs:
       - name: Install Dependencies
         run: npm ci
 
-      - name: Get composer cache directory
+      # This date is used to ensure that the Composer cache is refreshed at least once every week.
+      # http://man7.org/linux/man-pages/man1/date.1.html
+      - name: "Get last Monday's date"
+        id: get-date
+        run: echo "::set-output name=date::$(/bin/date -u --date='last Mon' "+%F")"
+        shell: bash
+
+      - 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' }}
         uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6 # v2.1.4
         env:
           cache-name: cache-composer-dependencies
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
-          key: ${{ runner.os }}-php-${{ matrix.php }}-composer-${{ hashFiles('**/composer.lock') }}
+          key: ${{ runner.os }}-php-${{ matrix.php }}-date-${{ steps.get-date.outputs.date }}-composer-${{ hashFiles('**/composer.json') }}
 
       - name: Install Composer dependencies
-        if: ${{ env.COMPOSER_INSTALL == true || env.LOCAL_PHP == '8.0-fpm' }}
         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.
+          # Install using `composer update` as there is no `composer.lock` file.
           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
+            docker-compose run --rm php composer update --ignore-platform-reqs
           else
-            docker-compose run --rm php composer install
+            docker-compose run --rm php composer update
           fi
 
       - name: Docker debug information
@@ -190,36 +188,36 @@ jobs:
 
       - name: Run slow PHPUnit tests
         if: ${{ matrix.split_slow }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ${{ env.SLOW_TESTS }}
+        run: npm run test:php-composer -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ${{ env.SLOW_TESTS }}
 
       - name: Run PHPUnit tests for single site excluding slow tests
         if: ${{ matrix.php < '7.0' && ! matrix.split_slow && ! matrix.multisite }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-required
+        run: npm run test:php-composer -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-required
 
       - name: Run PHPUnit tests for Multisite excluding slow tests
         if: ${{ matrix.php < '7.0' && ! matrix.split_slow && matrix.multisite }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-excluded,oembed-headers
+        run: npm run test:php-composer -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-excluded,oembed-headers
 
       - name: Run PHPUnit tests
         if: ${{ matrix.php >= '7.0' }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }}
+        run: npm run test:php-composer -- --verbose -c ${{ env.PHPUNIT_CONFIG }}
 
       - name: Run AJAX tests
         if: ${{ ! matrix.split_slow }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ajax
+        run: npm run test:php-composer -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ajax
 
       - name: Run ms-files tests as a multisite install
         if: ${{ matrix.multisite && ! matrix.split_slow }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c tests/phpunit/multisite.xml --group ms-files
+        run: npm run test:php-composer -- --verbose -c tests/phpunit/multisite.xml --group ms-files
 
       - name: Run external HTTP tests
         if: ${{ ! matrix.multisite && ! matrix.split_slow }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c phpunit.xml.dist --group external-http
+        run: npm run test:php-composer -- --verbose -c phpunit.xml.dist --group external-http
 
       # __fakegroup__ is excluded to force PHPUnit to ignore the <exclude> settings in phpunit.xml.dist.
       - name: Run (xDebug) tests
         if: ${{ ! matrix.split_slow }}
-        run: LOCAL_PHP_XDEBUG=true npm run test:${{ env.PHPUNIT_SCRIPT }} -- -v --group xdebug --exclude-group __fakegroup__
+        run: LOCAL_PHP_XDEBUG=true npm run test:php-composer -- -v --group xdebug --exclude-group __fakegroup__
 
       - name: Ensure version-controlled files are not modified or deleted
         run: git diff --exit-code
diff --git a/.github/workflows/test-coverage.yml b/.github/workflows/test-coverage.yml
index e82fa1e1e4..3c804c5ca6 100644
--- a/.github/workflows/test-coverage.yml
+++ b/.github/workflows/test-coverage.yml
@@ -18,9 +18,6 @@ on:
 
 env:
   PUPPETEER_SKIP_CHROMIUM_DOWNLOAD: ${{ true }}
-  COMPOSER_INSTALL: ${{ false }}
-  # Controls which NPM script to use for running PHPUnit tests. Options ar `php` and `php-composer`.
-  PHPUNIT_SCRIPT: php
   LOCAL_PHP: '7.4-fpm'
   LOCAL_PHP_XDEBUG: true
   LOCAL_PHP_MEMCACHED: ${{ false }}
@@ -88,6 +85,32 @@ jobs:
       - name: Install Dependencies
         run: npm ci
 
+      # This date is used to ensure that the Composer cache is refreshed at least once every week.
+      # http://man7.org/linux/man-pages/man1/date.1.html
+      - name: "Get last Monday's date"
+        id: get-date
+        run: echo "::set-output name=date::$(/bin/date -u --date='last Mon' "+%F")"
+        shell: bash
+
+      - name: Get Composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache Composer dependencies
+        uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6 # v2.1.4
+        env:
+          cache-name: cache-composer-dependencies
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-php-${{ matrix.php }}-date-${{ steps.get-date.outputs.date }}-composer-${{ hashFiles('**/composer.json') }}
+
+      - name: Install Composer dependencies
+        run: |
+          docker-compose run --rm php composer --version
+
+          # Install using `composer update` as there is no `composer.lock` file.
+          docker-compose run --rm php composer update
+
       - name: Docker debug information
         run: |
           docker -v
@@ -121,7 +144,7 @@ jobs:
 
       - name: Run tests as a single site
         if: ${{ ! matrix.multisite }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c phpunit.xml.dist --coverage-clover wp-code-coverage-single-clover-${{ github.sha }}.xml
+        run: npm run test:php-composer -- --verbose -c phpunit.xml.dist --coverage-clover wp-code-coverage-single-clover-${{ github.sha }}.xml
 
       - name: Ensure version-controlled files are not modified during the tests
         run: git diff --exit-code
@@ -135,7 +158,7 @@ jobs:
 
       - name: Run tests as a multisite install
         if: ${{ matrix.multisite }}
-        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c tests/phpunit/multisite.xml --coverage-clover wp-code-coverage-multisite-clover-${{ github.sha }}.xml
+        run: npm run test:php-composer -- --verbose -c tests/phpunit/multisite.xml --coverage-clover wp-code-coverage-multisite-clover-${{ github.sha }}.xml
 
       - name: Ensure version-controlled files are not modified during the tests
         run: git diff --exit-code