I have just updated my popular Optimizing PHP article with additional information on caching. I discuss memcache and squid. I also updated the PHP Accelerators and changed the tone of some parts of the article. I quote:
It is better to offload downloading of these static files by using Squid in httpd-accelerator mode. In this scenario, you use Squid as the front web server on port 80, and set all .php requests to be dispatched to Apache on port 81 which i have running on the same server. The static files are served by Squid.