how to use form_validation validation post data

post data to CI url,the post function below.
I sure url receive post data(log to file),but use form_validation validation args is invalid

PHP Code:

   private function http_post_data($url, $data_string, $authorization = null) {

        $_ch = curl_init();
        curl_setopt($_ch, CURLOPT_POST, 1);
        curl_setopt($_ch, CURLOPT_URL, $url);
        curl_setopt($_ch, CURLOPT_POSTFIELDS, $data_string);
        curl_setopt($_ch, CURLOPT_HTTPHEADER, array(
            'Content-Type: application/json; charset=utf-8',
            'Content-Length: ' . strlen($data_string),
            'Authorization:' . $authorization
                )
        );
        ob_start();
        curl_exec($_ch);
        $return_content = ob_get_contents();

        ob_end_clean();

        return $return_content;
    }

    public function post($url, $data) {
        $_data_string = json_encode($data);
        $_ret = $this->http_post_data($url, $_data_string);

        return json_decode($_ret);
    }