On the final hand, if you pass the pages off from apache to a php
exe or module.. How does Apache know which one to pass it to? Php4 or
Php5?
By the processing instruction target. That's what it's there for. I guess php 5 and 4 are not the best examples, since php5 already uses '<?php' PITarget. Let's say '<?php6' means PHP6, and '<?php' means 4/5.
If I have both php4 and php5 code in a page should i run both
sections or only the sections listed for php5(if it's on php5)?
You shouldn't run any sections, and give an error. The whole point is to separate different versions of the language instead of mixing them. Do you know a lot of scripts which use features specific to both php4 and php5? I don't. And it's an awful practice anyway.