A Lesson In Static Methods And Late Static Binding

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:

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1.

2. 

3.class My_Auth extends Zend_Auth

4.{}

5. 

6.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:

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01./**

02.* Returns an instance of Zend_Auth

03.*

04.* Singleton pattern implementation

05.*

06.* @return Zend_Auth Provides a fluent interface

07.*/

08.public static function getInstance()

09.{

10.if (null === self::$_instance) {

11.self::$_instance = new self();

12.}

13. 

14.return self::$_instance;

15.}

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.

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01.// Auth.php rewritten

02. 

03./**

04.* Returns an instance of Zend_Auth

05.*

06.* Singleton pattern implementation

07.*

08.* @return Zend_Auth Provides a fluent interface

09.*/

10.public static function getInstance()

11.{

12.if (null === static::$_instance) {

13.static::$_instance = new static();

14.}

15. 

16.return static::$_instance;

17.}

18. 

19.// My Auth class and sample code

20.class My_Auth extends Zend_Auth

21.{}

22. 

23.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.
The original work of Brandon Savage.