Inheritance, encapsulation, abstraction, and polymorphism are four fundamental concepts of object-oriented programming.
Object-oriented PHP
http://phpenthusiast.com/
Inheritance:
http://www.adobe.com/devnet/actionscript/learning/oop-concepts/inheritance.html
Encapsulation:
http://www.adobe.com/devnet/actionscript/learning/oop-concepts/encapsulation.html
Polymorphism and interface:
http://www.adobe.com/devnet/actionscript/learning/oop-concepts/polymorphism-and-interfaces.html
Interface:
http://php.net/manual/en/language.oop5.interfaces.php
