Go on
Hotscripts, download some scripts, set yourself up with a local web server and installation of PHP (use something like
WAMP to install everything with double-click ease, and delve into the code with an editor like
Winsyntax and start messing around and seeing which parts of code do what.
The easiest way to find this out is on the
PHP website, where you can type a function name in the search box to get a full description of what it does. Other than that, have fun with it!