Mongrel's plugin support will be available soon, and I'm going to take
a shot at getting a PHP interpreter working with that, but as for
right now, your best bet is to look up NanoWeb. It's a mini webserver
written in PHP, much like Webrick is to Ruby. It seems to perform very
well. If nothing else, you can configure it on a different port and do
some form of proxying to that port for getting the PHP pages served.

M.T.