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Access Specifiers
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Access Specifiers :

Access specifiers specify the level of access that the outside world (i.e. other class objects, external functions and global level code) have on the class methods and class data members. Access specifiers can either be public, private or protected. Access specifiers are used as a key component of Encapsulation and Data Hiding. By using either of the access specifiers mentioned above i.e. public, private or protected you can hide or show the internals of your class to the outside world.

1. Private :
A private access specifier is used to hide the data member or member function to the outside world. This means that only the class that defines such data member and member functions have access them.

    <?php
    class Customer {
	private $name;
	    public function setName($name) {
		$this->name = $name;
	    }
	    public function getName() {
		return $this->name;
	    }
    }
    $c = new Customer();
    $c->setName("Sunil Bhatia");
    echo $c->name;
    //error, $name cannot be accessed from outside the class
    //$name can only be accessed from within the class
    echo $c->getName();
    //this works, as the methods of the class have access
    //to the private data members or methods
    ?>

In the above example, echo $c->name will give you an error as $name in class Customer has been declared private and hence only be accessed by its member functions internally. Therefore, the following line echo $c->getName() will display the name.

2. Public :
A public access specifier provides the least protection to the internal data members and member functions. A public access specifier allows the outside world to access/modify the data members directly unlike the private access specifier.

    <?php
    class Customer {
	    public $name;
	    public function setName($name) {
		    $this->name = $name;
	    }
	    public function getName() {
		    return $this->name;
	    }
    }
    $c = new Customer();
    $c->setName("Sunil Bhatia");
    echo $c->name;          // this will work as it is public.
    $c->name = "New Name" ; // this does not give an error.
    ?>

In the above example, echo $c->name will work as it has been declared as public and hence can be accessed by class member functions and the rest of the script.

3. Protected :
A protected access specifier is mainly used with inheritance. A data member or member function declared as protected will be accessed by its class and its base class but not from the outside world (i.e. rest of the script). We can also say that a protected data member is public for the class that declares it and it's child class; but is private for the rest of the program (outside world).

    <?php
    class Customer {
	    protected $name;
	    public function setName($name) {
		    $this->name = $name;
	    }
	    public function getName() {
		    return $this->name;
	    }
    }
    class DiscountCustomer extends Customer {
	    private $discount;
	    public function setData($name, $discount) {
		    $this->name = $name; //this is storing $name to the Customer
		    //class $name variable. This works
		    // as it is a protected variable
		    $this->discount = $discount;
	    }
    }
    $dc = new DiscountCustomer();
    $dc->setData("Sunil Bhatia",10);
    echo $dc->name; // this does not work as $name is protected and hence
    // only available in Customer and DiscountCustomer class
    ?>

In the above example, echo $dc->name will not work work $name has been defined as a protected variable and hence it is only available in Customer and DiscountCustomer class.

In PHP5, access specifiers are public by default. This means that if you don't specify an access specifier for a data member or method then the default 'public' is applicable.