If you're a PHP developer and haven't made the jump to object-oriented programming yet, you definitely need to check out this extensive tutorial from thewebsqueeze.com. It's packed with plenty of OOP and wants to teach it all to you.

This tutorial aims to remove some of the curly-brace-filled haze that often surrounds OOP and by the end of this lesson you will have created a class, instantiated an object, given inheritance to some children and come to understand why OOP is such a powerful tool to have at your disposal. If you have no idea what any of the above means but are shaking with a desire to find out, read on as we look at Abstraction for the rest of us.

They work from the ground up - defining classes, setting properties with visibility, adding methods and a constructor, extending classes and using static methods/variables. They haven't updated it with the new namespace operator ("\" instead of "::") but other than that, it's a great overview. There's even some type checking and input filtering to round out the code examples.