PHP Singleton with child classes

gistfile1.aw

<?php

abstract class ParentClass {

    private static $instances = array();

    function getInstance($obj) {
        $class = get_called_class();

        if (!array_key_exists($class, self::$instances)) {
            self::$instances[$class] = new SplObjectStorage;
        }

        if (!self::$instances[$class]->contains($obj)) {
            self::$instances[$class]->attach($obj, new $class($obj));
        }

        return self::$instances[$class]->offsetGet($obj);
    }
}

class ChildClass1 extends ParentClass {}

class ChildClass2 extends ParentClass {}

$obj = new stdClass;

$inst1 = ChildClass1::getInstance($obj);
printf("I am %s, my class is %s\n", spl_object_hash($inst1), get_class($inst1));

$inst2 = ChildClass2::getInstance($obj);
printf("I am %s, my class is %s\n", spl_object_hash($inst2), get_class($inst2));

printf("Are we same object? %s\n", spl_object_hash($inst1) == spl_object_hash($inst2) ? 'yes' : 'NO!');