Use actions/cache for the Composer packages

ci/php.yml

@@ -12,8 +12,18 @@ jobs:
 
     - name: Validate composer.json and composer.lock
       run: composer validate
+      
+    - name: Cache Composer packages
+      id: composer-cache
+      uses: actions/cache@v1
+      with:
+        path: vendor
+        key: ${{ runner.os }}-node-${{ hashFiles('**/composer.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-node-
 
     - name: Install dependencies
+      if: steps.composer-cache.outputs.cache-hit != 'true'
       run: composer install --prefer-dist --no-progress --no-suggest
 
     # Add a test script to composer.json, for instance: "test": "vendor/bin/phpunit"