On the Google Developers Blog (and lots of places across the web) there's a major update that Google has done for their AppEngine service - the introduction of a PHP runtime to their offerings.
App Engine 1.8.0 is now available and includes a Limited Preview of the PHP runtime - your top requested feature. We’re bringing one of the most popular web programming languages to App Engine so that you can run open source apps like Wordpress. It also offers deep integration with other parts of Cloud Platform including Google Cloud SQL and Cloud Storage.
You can get more information about how to use this new feature on Google App Engine site.