PHP 5, Static and Inheritance

25 April 2007

The object oriented features of PHP 5 are leaps and bounds more complete than PHP 4. In fact, PHP 5 could almost pass for a true object oriented language in many ways: visibility, overloading, reflection, etc. Yet there are quite a few holes in its implementation that can make it painful to work with compared to similar languages. One well-known issue is that parent constructors are not implicitly called in derived classes; you must explicitly call parent::__construct().

I recently stumbled into a similar hole in PHP 5’s OO implementation: class methods and members declared as static do not behave as one might expect with respect to inheritance. Given a base class, Animal, and two derived classes, Dog and Cat, let’s assume that all Animals have a color and that it defaults to black.

<?php
class Animal
{
  protected static $color = 'Black';

  public static function getColor()
  {
    return self::$color;
  }
}

class Cat extends Animal
{
  protected static $color = 'Brown';
}

class Dog extends Animal
{
  protected static $color = 'Grey';
}
?>

Nice and simple, short and sweet. Animal has a color that defaults to black, and we’ve extended Animal with a Cat that defaults to brown and a Dog that defaults to grey. All fine and good. That is, until I want to ask the Dog or Cat classes what their default colors are.

<?php
echo Animal::getColor(); // Black
echo Dog::getColor();    // Black (huh?)
echo Cat::getColor();    // Black (what?)
?>

It turns out that static methods are called within the scope of the class where the method is defined. In this case, Animal. So, even though Dog and Cat both inherit the static method getColor(), its scope is bound to Animal.

The missing language feature that I want is called late static binding. The PHP Manual attempts to explain how things currently work.

… static method calls are resolved at compile time. When using an explicit class name the method is already identified completely and no inheritance rules apply. If the call is done by self then self is translated to the current class, that is the class the code belongs to. Here also no inheritance rules apply.

This issue has been a pain point for me lately, and I’ve resorted to some horrendous and clumsy workarounds to mimic the expected behavior. Until now there has been very little standing in my way of accomplishing most anything with PHP with relative ease. This is by far one of the most annoying dealings I’ve had with PHP in over 8 years.