Brian Nelson has posted a follow-up to his previous article on creating a reverse proxy with PHP5, this time expanding on the functionality of it and reworking some to increase performance.
It's gotten a bit more complex; The proxy handler didn't pass all the client headers to the proxy server. This caused problems with having the wrong client type, no Etag caching, cookie passing, etc. Here's the current rev, which solves a lot of these issues.
The complete code for the proxy class included in the post and a note was added that there's now a Google Code project that's been created for it (svn repository).