Add build steps for PHP 8 versions.

Author: chrisryan

.github/workflows/php.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['7.0', '7.1', '7.2', '7.3', '7.4']
+        php-versions: ['7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2']
     steps:
     - name: Checkout
       uses: actions/checkout@v2