无法发送cURL请求（POST）

Question

我尝试发送cURL请求以获取付款服务的响应。 cURL请求如下所示：

curl -X POST https://secure.payu.com/pl/standard/user/oauth/authorize \
-d 'grant_type=client_credentials&client_id=CLIENTID&client_secret=CLIENTSECRET'

我在Symfony的代码是：

 $ch = curl_init();

 curl_setopt($ch, CURLOPT_URL,"https://secure.payu.com/pl/standard/user/oauth/authorize");
 curl_setopt($ch, CURLOPT_POST, 1);
 curl_setopt($ch, CURLOPT_POSTFIELDS,
        http_build_query(array(
            "grant_type" => "client_credentials",
            "client_id" => $client_id,
            "client_secret" => $client_secret
        ))
 );
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

 $server_output = curl_exec ($ch);

 curl_close($ch);

 var_dump($server_output);

但我总是假的。我该怎么做才能发送卷曲请求并从付款服务中获得答案？