PHP self vs $this

Thursday, April 5, 2018

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:

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

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

new X();
?>

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self points to the class in which it is written.

So, if your getInstance method is in a class name MyClass, the following line :

self::$_instance = new self();

Will do the same as :

self::$_instance = new MyClass();

---------------------------------------

If you have two classes that extend each other, you have two situations :

getInstance is defined in the child class
getInstance is defined in the parent class

The first situation would look like this (I've removed all non-necessary code, for this example -- you'll have to add it back to get the singleton behavior)* :

class MyParentClass {

}
class MyChildClass extends MyParentClass {
    public static function getInstance() {
        return new self();
    }
}

$a = MyChildClass::getInstance();
var_dump($a);

Here, you'll get :

object(MyChildClass)#1 (0) { }

Which means self means MyChildClass -- i.e. the class in which it is written.

For the second situation, the code would look like this :

class MyParentClass {
    public static function getInstance() {
        return new self();
    }
}
class MyChildClass extends MyParentClass {

}

$a = MyChildClass::getInstance();
var_dump($a);

And you'd get this kind of output :

object(MyParentClass)#1 (0) { }

Which means self means MyParentClass -- i.e. here too, the class in which it is written.

With PHP < 5.3, that "the class in which it is written" is important -- and can sometimes cause problems.

That's why PHP 5.3 introduces a new usage for the static keyword : it can now be used exactly where we used self in those examples :

class MyParentClass {
    public static function getInstance() {
        return new static();
    }
}
class MyChildClass extends MyParentClass {

}

$a = MyChildClass::getInstance();
var_dump($a);

But, with static instead of self, you'll now get :

object(MyChildClass)#1 (0) { }

Which means that static sort of points to the class that is used (we used MyChildClass::getInstance()), and not the one in which it is written.

Of course, the behavior of self has not been changed, to not break existing applications -- PHP 5.3 just added a new behavior, recycling the static keyword.

And, speaking about PHP 5.3, you might want to take a look at the Late Static Bindings page of the PHP manual.