public function callback($callback)
    {
        $t = $this;
        $args = array_slice(func_get_args(),1);

        // use an event handler to dispatch to the callback
        $this->_events->register('startTransaction', function($event, $connection) use ($t, $callback, $args) {
            $t->results []= call_user_func_array($callback, $args);
        });

        return $this;
    }