protected function validate_valid_twitter($field, $input, $param = NULL)
    {
        if(!isset($input[$field]) || empty($input[$field]))
        {
            return;
        }
        $json_twitter = file_get_contents("http://twitter.com/users/username_available?username=".$input[$field]);
        
        $twitter_response = json_decode($json_twitter);
        if($twitter_response->reason != "taken"){
            return array(
                'field' => $field,
                'value' => $input[$field],
                'rule' => __FUNCTION__,
                'param' => $param
            );
        }
    }