在PHP中扩展静态类 - 避免在扩展类中为多个类共享变量

Question

I seem to have trouble extending static classes in PHP.

PHP Code:

<?php
    class InstanceModule {
        public static $className = 'None';
        public static function PrintClassName() {
            echo self::$className . ' (' . __CLASS__ . ')<br />';
        }
    }

    class A extends InstanceModule {
        public static function Construct() {
            self::$className = "A";
        }
    }

    class B extends InstanceModule {
        public static function Construct() {
            self::$className = "B";
        }
    }
?>

My calling code, and what I'd expect:

<?php
    //PHP Version 5.3.14

    A::PrintClassName(); //Expected 'None' - actual result: 'None'
    B::PrintClassName(); //Expected 'None' - actual result: 'None'

    A::Construct();

    A::PrintClassName(); //Expected 'A' - actual result: 'A'
    B::PrintClassName(); //Expected 'None' - actual result: 'A'

    B::Construct();

    A::PrintClassName(); //Expected 'A' - actual result: 'B'
    B::PrintClassName(); //Expected 'B' - actual result: 'B'

    A::Construct();

    A::PrintClassName(); //Expected 'A' - actual result: 'A'
    B::PrintClassName(); //Expected 'B' - actual result: 'A'
?>

Actual complete output:

None (InstanceModule)
None (InstanceModule)
A (InstanceModule)
A (InstanceModule)
B (InstanceModule)
B (InstanceModule)
A (InstanceModule)
A (InstanceModule)

So what's going on here (from what it seems) is that as soon as I set self::$className on either of the extending classes, it overrides the variable from the other class. I assume this is because I use static classes, and there can only be one InstanceModule class instead of simply copying it to both A and B, as were my previous understanding of extends. I've tried using the keyword static::$className instead, but it seems to make no difference.

It'd be lovely if anyone could guide me in the right direction of what I'm doing wrong here, and what to do to fix this problem.

Edit: To clarify, this code does what I want, but is obviously a horrible workaround since it would ruin the whole idea of extending and reusing functions:

<?php
    class A {
        public static $className = 'None';
        public static function PrintClassName() {
            echo self::$className . ' (' . __CLASS__ . ')<br />';
        }
        public static function Construct() {
            self::$className = "A";
        }
    }

    class B {
        public static $className = 'None';
        public static function PrintClassName() {
            echo self::$className . ' (' . __CLASS__ . ')<br />';
        }
        public static function Construct() {
            self::$className = "B";
        }
    }
?>

Answer 1

Since $className is static and within the parent class, when you set className within A or B, it changes the variable within the parent, and the same is done when the variable is read. Unless you override className in your extended classes, you'll be storing and retrieving information from the same memory location, originally defined in InstanceModule.

If you redefine className in A/B, you can access className using parent:: or self:: from InstanceModule or A/B respectively. Depending on what you are trying to do, Abstract classes may also play a significant role.

See Static Keyword or Class Abstraction on the PHP5 Manual.