Implement a PHP singleton: static class properties or static method variables?

So, I've always implemented a singleton like so:

class Singleton {
    private static $_instance = null;
    public static function getInstance() {
        if (self::$_instance === null) self::$_instance = new Singleton();
        return self::$_instance;
    }
    private function __construct() { }
}

However, it recently struck me that I could also implement it with member-wise static variables:

class Singleton {
    public static function getInstance() {
        //oops - can't assign expression here!
        static $instance = null; // = new Singleton();
        if ($instance === null) $instance = new Singleton();
        return $instance;
    }
    private function __construct() { }
}

To me, this is cleaner because it doesn't clutter the class, and I don't have to do any explicit existence check, but because I've never seen this implementation anywhere else, I'm wondering:

Is there anything wrong with using the second implementation over the first?