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:

<?php

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.

The original work of Brandon Savage.

Related posts:

1. Exceptional PHP: Extending The Base Exception Class
2. Exceptional PHP: Nesting Exceptions In PHP