On PHPMaster.com today there's a new tutorial introducing you to the PSR-0 standard and how it effects the autoloading in many PHP applications and frameworks. Specifically, they show how it's implemented in a Symfony2 component
In this article I’ll walk you through the “history of autoloading,” from the older to the current PSR-0 standard autoloader approach found in many PHP frameworks such as Lithium, Symfony, Zend, etc. Then I will introduce you to the ClassLoader component from the Symfony2 project for PHP 5.3 which follows the PSR-0 standard.
He starts with a look at a basic "__autoload" function call that looks in a directory for libraries. Improving on that, he makes two methods for loading - one for controllers, the other for models - and a loader that splits on the "_" character and determines the path from there.
Even this isn't PSR-0, though, so he shows how using namespace information, you can load classes in a unified way. He shows how to implement this with a loader that's already well-developed and ready for use - the Symfony ClassLoader component. They show how to register namespaces, prefixes as well as use the APC functionality to manually store/fetch the APC version of the loaded file's opcodes.