Extending singletons in PHP

I'm working in a web app framework, and part of it consists of a number of services, all implemented as singletons. They all extend a Service class, where the singleton behaviour is implemented, looking something like this:

class Service {
    protected static $instance;

    public function Service() {
        if (isset(self::$instance)) {
            throw new Exception('Please use Service::getInstance.');
        }
    }

    public static function &getInstance() {
        if (empty(self::$instance)) {
            self::$instance = new self();
        }
        return self::$instance;
    }
}

Now, if I have a class called FileService implemented like this:

class FileService extends Service {
    // Lots of neat stuff in here
}

... calling FileService::getInstance() will not yield a FileService instance, like I want it to, but a Service instance. I assume the problem here is the "self" keyword used in the Service constructor.

Is there some other way to achieve what I want here? The singleton code is only a few lines, but I'd still like to avoid any code redundance whenever I can.

Code:

abstract class Singleton
{
    protected function __construct()
    {
    }

    final public static function getInstance()
    {
        static $instances = array();

        $calledClass = get_called_class();

        if (!isset($instances[$calledClass]))
        {
            $instances[$calledClass] = new $calledClass();
        }

        return $instances[$calledClass];
    }

    final private function __clone()
    {
    }
}

class FileService extends Singleton
{
    // Lots of neat stuff in here
}

$fs = FileService::getInstance();

If you use PHP < 5.3, add this too:

// get_called_class() is only in PHP >= 5.3.
if (!function_exists('get_called_class'))
{
    function get_called_class()
    {
        $bt = debug_backtrace();
        $l = 0;
        do
        {
            $l++;
            $lines = file($bt[$l]['file']);
            $callerLine = $lines[$bt[$l]['line']-1];
            preg_match('/([a-zA-Z0-9\_]+)::'.$bt[$l]['function'].'/', $callerLine, $matches);
        } while ($matches[1] === 'parent' && $matches[1]);

        return $matches[1];
    }
}