NETTUTS.com has posted an introduction (using the Car analogy, of course) to teach you how to use the basics of object-oriented programming in PHP.

PHP is so much more than a scripting language. It's a full-fledged language capable of building very complex applications. By harnessing the full power of Object Oriented Programming, you can reduce the amount of time you spend coding and use it to build better websites. This tutorial will show you how.

They explain objects, inheritance, encapsulation and polymorphism briefly before "getting their hands dirty" with some actual OOP PHP code. Their code example sets up a thumbnailer class - one that takes in an image, a height/width you want to resize it to (as properties) and splits it back out the other side.

As always, you can grab the source or view the demo to see it work.