On the Builder AU site, there's this quick look at using the APC module (a PECL package) to help speed up your applications.

Looking to get every ounce of performance from your Web server? If you're using PHP scripts, you can easily accomplish this using PHP's APC module, which caches php opcode instead of forcing php to reinterpret every script every time it is executed.

The install is simple (on PHP5) and they show some configuration options you can use to fine-tune the performance of the module to fit your needs. There's also a brief look at what the module actually does to the scripts as they get interpreted and passed out of PHP and into the web server.