PHP 5: `$this` vs. `self` – When to Use Each?

Self vs. $this: When and How to Use Each

Question:

In PHP 5, how do the keywords "self" and "$this" differ in their usage? When should each be used appropriately?

Answer:

Short Answer:

Use "$this" to refer to the current object's instance variables and methods. Use "self" to refer to the current class's static variables and methods.

Full Answer:

Non-Static vs. Static Members:

• "$this->member" accesses non-static member variables and methods specific to the current object.
• "self::$member" accesses static member variables and methods shared by all instances of the class.

Polymorphism:

• "$this" can demonstrate polymorphism by calling methods defined in derived classes.
• "self" suppresses polymorphism and always calls the method defined in the current class.

Example (correct usage):

class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo $this->non_static_member . ' ' . self::$static_member;
    }
}

new X(); // Output: 1 2

Example (incorrect usage):

class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo self::$non_static_member . ' ' . $this->static_member; // Incorrect usage
    }
}

new X(); // Error: Undefined properties

Suppressing Polymorphism:

class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        self::foo(); // Suppresses polymorphism
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar(); // Output: X::foo()

Summary:

Use "$this" for non-static member access and polymorphism. Use "self" for static member access and when you need to suppress polymorphism.

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