Zend.com has another good article. This one is labeled Writing Libraries in PHP. This isn't a beginners tutorial, but does a good job of explaining how classes/libraries should be built.

Most of the documentation on PHP is focused on explaining the functioning of the various APIs PHP has available. Likewise, if you create a PHP library, most of the work involved is usually in sorting out how the API will work.

I particularly liked this article because it explains more of what a class should do for the programmer. There's also plenty of code examples to go around on this one. So if you're looking for a good explination or tutorial of classes and what they should do, check it out!