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.
To be fair, a good proportion of those comments were complaining about "+1" comments left on the issue. To an extent where there were more complaints than +1's.
Caucho says that the real gain is taking advantage of built-in advanced features of Resin App Server like load balancing. Without porting MySql, PHP on GAE means little.
Perhaps because they rely on a lot of open source components and open up some of their technologies whereas Microsoft has been slow at coming around to the OS way of thinking.
If you're worried about lock-in with App Engine, check this out: http://code.google.com/p/appscale/ As you can see, Google is one of the supporters of this project, although it's not clear exactly what they have contributed.
Deploying onto the JVM is hardly vendor lock-in; there are no shortage of Java servers around, from a variety of vendors. Obviously the specifics of deploying to App Engine are going to be vendor-specific, but the core of your code should be pretty portable.
Anyone here has experience with Quercus? How compatible is it with PHP? Do apps like Wordpress or Drupal run out of the box? Any noticeable performance difference?
Quercus claims to be able to run "MediaWiki, Drupal, Wordpress, Gallery2 and DocuWiki" on their site, and equivalent performance to a PHP deployment with APC. Which is pretty impressive, if true.
Hmmm, I just looked on my Google App Engine account, and I can't see how to use / enable PHP. I see the link to quercus, ... can someone add some details to how to use it?
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.