For those working to move from procedural PHP into a more object-oriented world but may be having some trouble with the transition, the Star Tutorial site has a great beginner OOP in PHP guide you should check out.
They cover all of the basics you'll need to get started with objects in PHP including:
- classes versus objects
- interfaces versus abstract classes
Each section is a quick definition and a bit of code to help illustrate the point. This isn't going to be a hand-holding kind of tutorial showing you each step to making an OOP application. Instead, it provides quick, high level summaries of the main OOP concepts to get you on the right road.