How can a clone can create a new object of 'Singlton' class

I asked this question and got a good solution. And i understand that how an object can be created without new (which i thought the only way to construct an object).

Now i was studying the Design patterns and learned that by creating a Singlton class we force a class to instantiate only once.

// a simple Singlton class.
class Singleton
{
    private static $object = null;
    private function __construct() {}
    public static function createObject()
    {
        if (self::$object == null) {
            self::$object = new Singleton();
        }
        return self::$object;
    }
}

Now i instantiate this class.

//$obj = new Singleton(); Obviously an error

$obj1 = Singleton::createObject();
// object(Singleton)[1]

$obj2 = Singleton::createObject();
// object(Singleton)[1]
// $obj1 and $obj2 are same. Both have same id = 1

$obj3 = clone $obj1;
// object(Singleton)[2]
// $obj3 is a new  instantiate. id = 2

So my question is how is this happening. how does the clone work here.
i wish php have an singlton keyword to create singlton class. :)