Sameer Borate has posted a guide to his site today showing you how to get started with PHP applications on the Google App Engine now that it natively supports it (well, mostly).
A couple of years back if you needed to run PHP on Google App Engine you were required to use a open source tool like Quercus, a 100% Java implementation of PHP, to run your PHP applications on the App Engine. However, as you would have guessed, it was not easy to work as with using a native PHP implementation. Now that App Engine natively supports PHP and MySQL, you can easily write PHP applications.
He walks you through the setup and configuration of the App Engine SDK to interact with the service and shows how to create a simple "Hello World" PHP application and deploy it. He talks some about the overall advantages of using the Google App Engine for your application including the scalability it offers and the secured infrastructure it runs on. He finishes the post looking at the different options for data storage and how sessions should be handled (hint: memcache).