private function isValidCallback($callback)
    {
        // For security purposes, we don't want to call anything that isn't
        // an object callback
        if (is_string($callback)) {
            return false;
        }

        if (is_array($callback)) {
            $target = array_shift($callback);
            if (! is_object($target)) {
                return false;
            }
        }

        // Object callback -- always okay
        return true;
    }