Encapsulation:
The wrapping of data and function together in a single unit(class) is called encapsulation.
In PHP 4 objects were little more than arrays.In PHP 5 you get much more control by visibility, 
interfaces,and more.


PHP5 provides data-hiding capabilities with public, protected, and private data attributes and methods:


Public : A public variable or method can be accessed directly by any user of the class.


Protected : A protected variable or method cannot be accessed by users of the class but can be accessed inside a subclass that inherits from the class.


Private: private variable or method can only be accessed internally from the class in which it is defined.


E.g.:
class is a protective wrapper which binds data and methods together,they can be accessed only though object of that class.


Example:


<?php 


class Shape { 


     private static $_circle; 


     public function Circle( ) { 
          if( $this->_circle == null ) { 
               $this->_circle = new Circle(); 
          } 
          return $this->_circle; 
     } 


} 


class Circle { 


     private $_radius; 


     public function __construct() { 
          $this->_radius = "10"; 
     } 


     public function GetRadius() { 
          return $this->_radius; 
     } 
} 


$shape= new Shape(); 


echo $shape->Circle()->GetRadius(); 
?>