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