CURL: pasar parámetros POST con http_build_query

$fp = fopen('curl.txt', 'a');

$campos = array(
    'campo1' => 'xxx',
    'campo2' => 'xxx',
    'campo3' => 'xxx',
   'campo4' => 'xxx'
  );


  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($ch, CURLOPT_HEADER, false);
  curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/x-www-form-urlencoded'));
  curl_setopt($ch, CURLOPT_POST, true);
  curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($campos));
  curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
  curl_setopt($ch, CURLOPT_VERBOSE, true);
  curl_setopt($ch, CURLOPT_STDERR, $fp;);
  $server_output = curl_exec($ch);

if ($server_output === false) {
    exit("Error Number: " . curl_errno($ch) . "<br>Error String: " . curl_error($ch) . "<br>Info: " . print_r(curl_getinfo($ch), true));
  } else {
    $res = json_decode($server_output);
  }
  curl_close($ch);