Google recently announced the addition of PHP to the Google App Engine; a move that makes sense with PHP being one of the most popular programming language nowadays. However, according to Google, the PHP runtime environment for App Engine “is an Experimental feature in Limited Preview.” This means that while anyone who wants to can develop and test applications locally, “only whitelisted applications may be deployed on App Engine if they use the PHP runtime”. Of course, this restriction will be lifted eventually, although when that will be exactly is yet to be announced. Google is accepting applications from developers who wish to be able to deploy their programs on App Engine, although sending in your application is not an insurance that you will indeed be whitelisted.
If you want to know more about using PHP to develop programs on Google’s App Engine, you can read the PHP Getting Started Guide.