Is there a rationale for doing this, other than "because it's there"?
AFAIK the problem with running existing PHP apps on App Engine isn't the PHP part. It's the rest of their architecture. I've only ever seen two kinds of PHP apps: The ones that are tightly coupled to the LAMP stack or a very close equivalent, and the ones that should have been written in some other language.
I'm not sure about GAE, but as to Quercus in general: You get to write in Not Java while your employer gets to deploy on the JVM.
This is huge.
Take Jython for example: Put jython.jar in WEB-INF/lib/, add a small stanza to WEB-INF/web.xml, write a WSGI-compliant app in Python. When you're done, you can zip it up and deploy it on Tomcat. None of the existing infrastructure needs to change, and you get to write in a more expressive language. That's how we'll sneak better things into enterprise.
AFAIK the problem with running existing PHP apps on App Engine isn't the PHP part. It's the rest of their architecture. I've only ever seen two kinds of PHP apps: The ones that are tightly coupled to the LAMP stack or a very close equivalent, and the ones that should have been written in some other language.
But maybe someone else can enlighten me.