Instance

What is an instance?
http://whatis.techtarget.com/definition/instance 

An instance, in object-oriented programming (OOP), is a specific realization of any object. An object may be varied in a number of ways. Each realized variation of that object is an instance. The creation of a realized instance is called instantiation.

Each time a program runs, it is an instance of that program. In languages that create objects from classes, an object is an instantiation of a class. That is, it is a member of a given class that has specified values rather than variables. In a non-programming context, you could think of "dog" as a class and your particular dog as an instance of that class.

OOP programming links

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