Late Static Binding in PHP

I have just sent an e-mail to the internals list for PHP that implements late static binding. Many of you probably remember hearing about all of this a few months ago not too long after the Zend Framework webcast where they showed off an API that appeared to be somewhat impossible to create short of some poor hacks with terrible performance.

A good summary of the problem this patch fixes can be found on Joshua Eichorns blog: http://blog.joshuaeichorn.com/archives/2006/01/09/zactiverecord-cant-work/

So, for a more detailed summary of why this doesn't work yet.

Take the following code:

<?php
class Foo {
    protected static $staticVar = 'Foo';
    public static function test() {
        echo self::$staticVar . "\n";
    }
}
Foo::test();

class Bar extends Foo {
    protected static $staticVar = 'Bar';
}
Bar::test();
?>

The above code seems fairly straight forward. What happens behind the scenes is that a Foo class is created and has the static function added to it's function table (among other things.) At that time the class Foo is also added to the Foo::test function struct that is added to that table. So, the when Foo::test() is called, the engine looks for the Foo class, finds the test function in Foo's function table and executes it.

So, when Bar is defined, it creates the Bar class. Then it notices that Bar extends foo and adds all of Foo's functions, properties, and what not to the function table, property table, etc. Now, when this is done no additional changes are made to the Foo::test function struct to show that there is a new class (Bar) that can call it. So when you call Bar::test() it will find Bar, and find Foo::test but when the test function is ran no information is passed to the function to let it know that Bar was the class that called it. To put it more succinctly, Foo and Bar both know about ::test, but ::test has no idea who Bar is. The function will always execute in Foo's class scope. So self:: always calls Foo::. Now, this isn't always a problem. The only time it becomes an issue is if you have an inherited static function that wants to call another static function, static property, or constant that has been redefined in a child class.

The change I made basically allows the engine to store the class calling the static function. The caller can then be accessed using 'static' (as opposed to self.) This should open the door to many other framework implementation possibilities (First thing that comes to mind for me is a Rubyish Active Record object.)

So, if you take part in the PHP-DEV mailing list and you want late-static-binding let them know.