On the PHPro.org website, Kevin Waterson has written up an introduction to the handy autoloading functionality that comes with PHP5 releases - the __autoload method from the Standard PHP Library (SPL).
The SPL __autoload() method is one of the Magic Methods supplied in PHP. The __autoload method is called whenever a class is instantiated and will load the classs the the first time it is called. No longer is include(), require, include_once() or require_once needed as the SPL autoload takes care of this interally.
He looks at how you can use it to load a directory of classes, how to use multiple autoloads in a single script and how to use it with interfaces to register a loader and include it as needed.