public function toXMLString()
{
$str = '';
$str .= '<ns:BillUserReq>';
if ($this->BillUserRequest != null) {
$str .= '<ns:BillUserRequest>';
$str .= $this->BillUserRequest->toXMLString();
$str .= '</ns:BillUserRequest>';
}
$str .= '</ns:BillUserReq>';
return $str;
}