public function bindService($runlevel, $service)
    {

        // bind the service to the runlevel
        $this->runlevels[$runlevel][$service->getName()] = $service;

        // bind the service callback to the naming directory
        $this->getNamingDirectory()->bindCallback(
            sprintf('php:services/%s/%s', $this->runlevelToString($runlevel), $service->getName()),
            array(&$this, 'getService'),
            array($runlevel, $service->getName())
        );
    }