Creating a Singleton base class in PHP 5.3

Question

Straight to the point: I've got two singleton classes, both inheriting their singleton nature from a super-class. I initialize some properties on the first singleton, and then have the second singleton retrieve the instance of the first one. That instance, however, does not seem to be the one I initialized in the first place. Some example code might help to explain this:

First, the super-class, providing singleton nature (requires PHP 5.3 or greater):

class Singleton {

    protected static $instance;

    protected function __construct() { }

    final private function __clone() { }

    public static function getInstance() {
        if (!(static::$instance instanceof static)) {
            static::$instance = new static();
        }
        return static::$instance;
    }

}

Then we've got the the first singleton carrying a value:

require_once('Singleton.php');

class SingletonA extends Singleton {

    protected $value;

    public function SingletonA() {
        $this->value = false;
    }

    public function getValue() {
        return $this->value;
    }

    public function setValue($value) {
        $this->value = $value;
    }

}

Then the second singleton that references the first singleton:

require_once('Singleton.php');
require_once('SingletonA.php');

class SingletonB extends Singleton {

    public function getValue() {
        return SingletonA::getInstance()->getValue();
    }

}

Now for the test that shows how this fails:

require_once('SingletonA.php');
require_once('SingletonB.php');

SingletonA::getInstance()->setValue(true);

echo (SingletonA::getInstance()->getValue()) ? "true\n" : "false\n";
echo (SingletonB::getInstance()->getValue()) ? "true\n" : "false\n";

The test yields the following output:

true
false

Clearly, the SingletonA instance that the test code references is not the same instance that the SingletonB instance references. In short, SingletonA is not as single as I need it to be. How is this possible? And what magic can I wield to remedy this behaviour, giving me a true singleton?

Answer 1

Try using isset rather than instanceof:

class Singleton {
    protected static $instances;

    protected function __construct() { }

    final private function __clone() { }

    public static function getInstance() {
        $class = get_called_class();

        if (!isset(self::$instances[$class])) {
            self::$instances[$class] = new $class;
        }
        return self::$instances[$class];
    }
}