how php 5 works

Webmaster, I've done a bit of looking, but have not found a description of how PHP 5 and the Apache PHP module work together. I'm curious about the depth of compilation/parsing when the script is first loaded vs delayed parsing until a method might be called. I'm also curious about caching of objects that takes place by the PHP module. Is there a paper or article describing this? Having some understanding I find can improve the code I write. Thanks in advance, SRG