public function header($namespace, $name, $data = null, $mustUnderstand = false, $actor = null)
  {
    if ($actor) {
      $this->headers[] = new SoapHeader($namespace, $name, $data, $mustUnderstand, $actor);
    } else {
      $this->headers[] = new SoapHeader($namespace, $name, $data, $mustUnderstand);
    }

    return $this;
  }