This move doesn't make much sense to me. Most of the websites built in PHP are huge CMS services like Weebly, Wordpress, Joomla, or old social networks like Facebook. I don't see any of these large services switching over to get hosted on Google. Plus PHP in general is disorganized and slow compared to more contemporary languages with well defined frameworks.
In addition to those hosted services, is all of the self-hosted Wordpress, Joomla, Zencart, etc. Plus all of the business and personal sites built on Zend, Symfony, Cake.
And most of those Wordpress/etc installs are going to be of applications which assume they're in a normal hosting environment, where they're running on Apache with a filesystem they can write to, connecting to a MySQL server, etc. How well will this translate to GAE? I'm not sure.
