As of Google IO this year, Google App Engine officially supports PHP. While it’s still in beta, you can tweak your applications and give it a try right now. I’ve included some of my lessons learned from getting Twilio’s quickstarts functional on GAE.
You can read the results here: Getting started with Twilio on Google App Engine for PHP.
And I also have a followup being published with php|Architect in the next month or two.