As I said in my welcome post, I have uploaded a series of lessons i wrote on the uses of OOP in PHP. The body of the lessons was about creating an application that will mimic Richard Dawkins' mutation program, which he describes in his book The Blind Watchmaker. Brought to you here is a table of content for these lessons:

1. An Introduction to Object Oriented Programing -  In this article we entered the basics of OOP - basic usage and terminology, visibility, inheritance, and the use of constructors.
2. OOP Tutorial Part 1 - In this post we started a 3 - part series, taking our first steps creating an OOP application. Other than creating our project's outline and our first class, we also learned about interfaces and dependency injection.
3. OOP Tutorial Part 2 - In this post we wrote our Tree class. We became slightly familiar with the GD library, and with implementing the principles of dependency injection.
4. OOP Tutorial Part 3 - In this post we wrapped up our application with the Garden class, that gave us an interface with which to interact with our classes.

Other Useful links:

1. A running demo of our application.
2. Source code for the demo on github.
3. A link to the book's wikipedia page.
4. A much much better implementation of our program
5. OOP in PHP documentation. Recommended you take the time and read it all. It's very enlightening.