php curl发送POST application/json请求

    public function curl2($url, $data)
    {
        $curl = curl_init();
        curl_setopt($curl, CURLOPT_URL, $url);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, false);
        if (!$data) {
            return 'data is null';
        }
        if (is_array($data)) {
            $data = json_encode($data);
        }
        curl_setopt($curl, CURLOPT_POST, 1);
        curl_setopt($curl, CURLOPT_POSTFIELDS, $data);
        curl_setopt($curl, CURLOPT_HEADER, 0);
        curl_setopt($curl, CURLOPT_HTTPHEADER, array(
            'Content-Type: application/json; charset=utf-8',
            'Content-Length:' . strlen($data),
            'Cache-Control: no-cache',
            'Pragma: no-cache'
        ));
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        $res = curl_exec($curl);
        $errorno = curl_errno($curl);
        if ($errorno) {
            return $errorno;
        }
        curl_close($curl);
        return $res;