php - When to use self over $this?

In PHP 5, what is the difference between using self and $this?

When is each appropriate?

Answer

Use $this to refer to the current
object. Use self to refer to the
current class. In other words, use
$this->member for non-static members,

use self::$member for static members.

Here is an example of correct usage of $this and self for non-static and static member variables:

class X {
    private $non_static_member = 1;

    private static $static_member = 2;

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

new X();
?>

Here is an example of incorrect usage of $this and self for non-static and static member variables:

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

    function __construct() {

        echo self::$non_static_member . ' '
           . $this->static_member;
    }
}

new X();
?>

Here is an example of polymorphism with $this for member functions:

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

    function bar() {
        $this->foo();
    }

}

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

$x = new Y();
$x->bar();

?>

Here is an example of suppressing polymorphic behaviour by using self for member functions:

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


    function bar() {
        self::foo();
    }
}

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

}

$x = new Y();
$x->bar();
?>

The idea is that $this->foo() calls the foo() member function of whatever is the exact type of the current object. If the object is of type X, it thus calls X::foo(). If the object is of type Y, it calls Y::foo(). But with self::foo(), X::foo() is always called.

From http://www.phpbuilder.com/board/showthread.php?t=10354489:

By http://board.phpbuilder.com/member.php?145249-laserlight