Add PHPUnit test action

.github/workflows/phpunit.yml

@@ -0,0 +1,35 @@
+name: PHPUnit tests
+on:
+  push
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php: [ '8.1', '8.2', '8.3', '8.4' ]
+    name: On PHP ${{ matrix.php }}
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup PHP ${{ matrix.php }}
+      uses: shivammathur/setup-php@v2
+      with:
+        php-version: ${{ matrix.php }}
+
+    - name: Install dependencies (latest)
+      run: composer install --prefer-dist --no-progress
+
+    - name: Run test suite (latest)
+      run: ./vendor/bin/phpunit
+
+    - name: Install dependencies (lowest)
+      run: composer update --prefer-lowest --no-progress
+
+    - name: Run test suite (lowest)
+      run: ./vendor/bin/phpunit