From the DeveloperTutorials.com website today there's a new walk-through showing you how to get PHP5 installed with Apache and MySQL onto a linux machine of your choosing. They chose to go with SuSE, but it should work on just about anything else.
We will set up PHP as a shared module, being loaded into Apache2 dynamically during the server startup. These instructions are known to work for PHP versions: 5.0.4 through 5.2.1.
They list a few prerequisites (the obvious ones) and where to grab the source for PHP. The rest is broken up into a few simple (hopefully) steps for the unpacking, compiling, configuring and integration with Apache.