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Until last week, I had never experienced what must have been incredibly frustrating to most developers: the fact that the self keyword in PHP refers to the class it is located in, and not necessarily a class that extends it. I personally ran into this problem when trying to extend Zend_Auth. Being a singleton, the constructor in Zend_Auth is protected, and the static method Zend_Auth::getInstance() instantiates itself. The problem is, when extended, My_Auth::getInstance() still returns an instance of Zend_Auth. The solution was to duplicate the static method in my My_Auth class, which worked properly. For example:
<?php <span id="more-1264"></span> class My_Auth extends Zend_Auth {} echo get_class(My_Auth::getInstance());
What did I get as a return value? Zend_Auth – because here is the source code of the getInstance() method in Zend Framework’s Zend_Auth class:
/** * Returns an instance of Zend_Auth * * Singleton pattern implementation * * @return Zend_Auth Provides a fluent interface */ public static function getInstance() { if (null === self::$_instance) { self::$_instance = new self(); } return self::$_instance; }
Why didn’t I get an instance of My_Auth instead of Zend_Auth? Well, that’s because PHP determines the meaning of the self keyword at compile time, meaning that when you call a function that makes use of it later, you’ll get whatever it’s been defined to mean when it was compiled.
PHP 5.3 provides a workaround for this, called late static binding. Simply put, PHP 5.3 introduces a new use of the keyword static, that allows you to avoid this define-at-compile-time problem. Using PHP 5.3 with the same example, and the static keyword, here is what happens.
// Auth.php rewritten /** * Returns an instance of Zend_Auth * * Singleton pattern implementation * * @return Zend_Auth Provides a fluent interface */ public static function getInstance() { if (null === static::$_instance) { static::$_instance = new static(); } return static::$_instance; } // My Auth class and sample code class My_Auth extends Zend_Auth {} echo get_class(My_Auth::getInstance());
The result here is now that an instance of My_Auth is returned. Late static bindings make working with static methods much easier. For this feature alone I believe PHP 5.3 is a worthwhile upgrade.
Brandon Savage is the author of Mastering Object Oriented PHP and Practical Design Patterns in PHP
Posted on 4/12/2010 at 7:00 am
James wrote at 4/12/2010 11:53 am:
Just though I would point out the get_called_class function. It provides access to the name of the called class, not just a reference to it:
http://us.php.net/manual/en/function.get-called-class.php
You can think of it as the late-static-binding equivalent to __CLASS__.
Gargoyle wrote at 4/12/2010 4:32 pm:
Nice and simple way to show an example of LSB…
till (@klimpong) wrote at 4/13/2010 6:26 am:
Great example, but better yet an example of why singletons are to be avoided – always.
Sebs wrote at 4/14/2010 7:19 am:
I wonder what kind of language this is, where we need stuff like that. There is a Quadrupzillion Solutions that wont hurt your mind when you think about em. This is what OOP and PHP where about in the first place.
If someone says to me: late static binding, my brain goes into a “does not compute” mode and I ask the person to explain the problem that needs to be solved. If the solution is a technical solution that only solves a problem that another technical solution brought up …. you either see the recursion already or you learn late static binding.2 cents
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