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constructor:
============
Constructor is a magic method which is invoked at the time of creating an instance of a class.
in php5 it is __construct(). In php4 it is the name of the class itself

destructor:
============
destructor is a magic method which is invoked at the time of destruction of an instance of a class.
In php5 it is __destruct()

Inheritance:
============
Inheritance is an oops concept. when a parent class has a public or protected method or property, it is available in its child class without defining inside child class.

static:
=======
if a method is defined as static, it can be called without creating an instance of the class using scope resolution operator ::

class constants are always statically called.

autoloading classes:
====================
__autoload is a magic method. This method is called by php when a non existent classes in a php script.
The name of the class is sent as an argument to this method and it can include the class. reference: http://www.php.net/manual/en/language.oop5.autoload.php

visibility:
===========
public: properties and methods are visible everywhere.
protected: properties and methods are visible at the same class and child classes
private: properties and methods are visible at the same class alone.

if you try to access protected / private property or method in the instance of the class like the following, you will get fatal error
echo $obj->protected; // Fatal Error
echo $obj->private; // Fatal Error

refernce: http://www.php.net/manual/en/language.oop5.visibility.php

abstract class:
===============
you cannot create instance for abstract class.
you can extend abstract class and create instance for the extending class
All the methods marked as abstract should be defined in the extending class
all the visible methods that are not marked as abstract are available in the extending class (child class) 

Interfaces:
===========
Interfaces are a type of abstract classes.
In interfaces you cannot define methods. Only function declaration without function body is allowed.
All the methods declared in an interface should be defined by the class implementing that interface
A class can implement multiple interfaces (This effectively implements multiple inheritance in php)

overloading:
============

creating methods and properties at runtime is called property overloading / method overloading.
In php, property overloading can be done by magic methods like __set, __unset, __isset, __get
method overloading can be done by magic methods like __call and __call_static

                                             
magic methods
=============
magic methods are functions that are call not by the user by php itself at runtime at certain events.
examples: __call, __set, __unset, __autoload, __construct, __destruct, __sleep, __wakeup etc.

polymorphism:
============
polymorphism means "having multiple forms"
a same method or property can behave differently at different situations.
for example, animal class can have a method called speak().
when a class called cat extends animal, the class will return "meow" when you call speak()
when a class called dog extends animal, the class will return "wow wow!!" when you call speak()
etc




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