public function setCallback($callback)
    {
        if (is_array($callback)) {
            require_once 'Zend/Server/Method/Callback.php';
            $callback = new Zend_Server_Method_Callback($callback);
        } elseif (!$callback instanceof Zend_Server_Method_Callback) {
            require_once 'Zend/Server/Exception.php';
            throw new Zend_Server_Exception('Invalid method callback provided');
        }
        $this->_callback = $callback;
        return $this;
    }