CURL not working correct by including script

Hello WordPress!

I’am trying to make a plugin for WordPress. This plugin is based on a script that was before running on its own. Now I am trying to include this script to make it a plugin.

Now everything is working except a function where i’am using cURL. I get the following error:

Warning: curl_setopt(): supplied argument is not a valid cURL handle resource in /home/*******.php on line 16
and the lines after this.

This is the code:

$ch = curl_init();

  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_COOKIEFILE, "cookie.txt");
  curl_setopt($ch, CURLOPT_COOKIEJAR, "cookie.txt");
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  curl_setopt($ch, CURLOPT_POST, 1);   

  $postdata=array(
    "name" => $Mail,
    "pass" => $Wachtwoord,
    "form_id" => "user_login",
    "op" => "Log in",
  );   

  curl_setopt ($ch, CURLOPT_POSTFIELDS, $postdata);   

  $result=curl_exec($ch);

So the works fine outside of WordPress but including it in a plugin does not work.

Can anybody help me?