Add PHPUnit testing framework and error checking documentation

.github/workflows/phpunit.yml

@@ -0,0 +1,61 @@
+name: PHPUnit Tests
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    name: PHP ${{ matrix.php }} - WP ${{ matrix.wp }} - ${{ matrix.multisite && 'Multisite' || 'Single Site' }}
+    runs-on: ubuntu-latest
+    
+    strategy:
+      matrix:
+        php: [ '7.4', '8.0' ]
+        wp: [ 'latest' ]
+        multisite: [ false, true ]
+    
+    services:
+      mysql:
+        image: mysql:5.7
+        env:
+          MYSQL_ROOT_PASSWORD: root
+          MYSQL_DATABASE: wordpress_test
+        ports:
+          - 3306:3306
+        options: --health-cmd="mysqladmin ping" --health-interval=10s --health-timeout=5s --health-retries=3
+    
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+      
+      - name: Setup PHP
+        uses: shivammathur/setup-php@e6f75134d35752277f093989e72e140eaa222f35 # v2.30.0
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: dom, curl, libxml, mbstring, zip, pdo, mysql, pdo_mysql, bcmath, soap, intl, gd, exif, iconv
+          coverage: none
+      
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@83af392bf5f031813d25e6fe4cd626cdba9a2df6 # v2.2.0
+      
+      - name: Install WordPress test suite
+        run: |
+          bash bin/install-wp-tests.sh wordpress_test root root 127.0.0.1 ${{ matrix.wp }} ${{ matrix.multisite && 'true' || 'false' }}
+      
+      - name: Run PHPUnit tests
+        run: |
+          if [ "${{ matrix.multisite }}" = "true" ]; then
+            WP_MULTISITE=1 composer test
+          else
+            composer test
+          fi