I've already been developing in App Engine using Java for a lot of times but it was only in Google I/O 2013 Extension, Manila when I first heard of the Google App Engine for PHP. That was the launching! So, I immediately signed up for being a beta/charlie tester.
Today is a lucky day! I got an email saying my app got whitelisted for PHP deployment. Yipee! But it really took a while. I think it has something to do with the new version release. Anyway, I'll probably gonna try putting WordPress. :-)