curl 'https://XXXXXXXX/login' -H 'Authorization: Basic XXXXXXXX=='
      -H 'Content-Type: application/x-www-form-urlencoded'
      --data 'username=XXXXXXXX&password=XXXXXXXX'

class XXXXXX {
      function login($username='', $password='') {
        // construct url - returns https://XXXXXXXX/login
        $url = $this->constructLoginUrl();

        $data = array(
          'username' => $username,
          'password' => $password
        );

        $headers = array(
          'Authorization' => "Basic XXXXXXXX==",
          'Content-Type'  => "application/x-www-form-urlencoded"
        );

        $response = drupal_http_request( $url, $headers, 'POST', http_build_query( $data )

        return $response;
      }
    }

drupal_http_request($url, array(
  'headers' => $headers,
  'method' => 'POST',
  'data' => http_build_query($data)
));