Actually, it's funny you should mention that. I wrote a little web server in PHP that's basically a port of Ruby's Rack:

http://github.com/dhotson/kelpie

It actually works pretty well, no apache required! It's pretty fast too.. all the init stuff you normally have to do every single request can be done once on server startup.

I also wrote a little sinatra style framework to go on top of it: http://github.com/dhotson/kelpie/blob/master/demoapp.php

Anyway, it's turning out to be a pretty fun little project. :-)