Google's rollout processes are very slow sometimes. PHP support in Google App Engine was launched back in 2013 — but it's only now that the product is officially out of beta and "generally available".
The main practical benefit of this, as Google's announcement post points out, is that the service level agreements for App Engine now apply to the PHP version as well. That lack of a formal agreement hasn't stopped the service being embraced; Google says it handles 800 million PHP queries a week via App Engine.
App Engine for PHP is generally available. Build scalable apps, faster. [Google Cloud Platform Blog]