/**
     * Invoke method by using reflection.
     * Require PHP5 (>= 5.3.2)
     * @param $method_name
     * @param $obj - An object that invokes the method.
     * @param mothod_args... Arguments for the method.
     */
    public static function invoke_method($method_name, $obj) {
        $class_name = get_class($obj);
        $class = new ReflectionClass($class_name);
        $method = $class->getMethod($method_name);
        $method->setAccessible(true);

        $mothod_args = func_get_args();
        array_shift($mothod_args);
        array_shift($mothod_args);

        return $method->invokeArgs($obj, $mothod_args);
    }