New from WebReference.com there's the first part of a tutorial series looking at working with object-oriented programming in PHP:

It is essentially a different way of constructing your applications. Instead of having loose procedures and functions that are meant to serve one purpose, Object Orientated Programming enables you to put data and functions in one container. This container is referred to as an object. An object enables you to model your application as closely as possible to the real world.

They introduce the object - what it is, some of its properties - and include some example code to show how it works. Their sample defines a class and shows how to call methods and set properties on it. They also touch on some of the OOP basics like constructors and inheritance,