Update workflow to run on PHP 7.2

.github/workflows/php.yml

@@ -9,9 +9,15 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
+      - name: "Set up PHP"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          php-version: "7.2"
+          coverage: "none"
+    
       - uses: actions/checkout@v3
 
       - name: Validate composer.json and composer.lock