XML 数据过早结束

运行下面的代码时，我收到以下错误：

加载 XML 失败 标记中的数据过早结束 请求行 2

致命错误：在第 61 行的/home4/viptrav3/public_html/wp-content/themes/voyage-child/transfer.php 中的非对象上调用成员函数 asXml（）

61号线$requestFile->asXml('trequest.xml');

但我没有看到 XML 过早关闭。我想将其另存为 XML 文件 trequest.xml

//Build XML Request
                $requestData = '<?xml version="1.0" encoding="UTF-8" ?>' . "'n";
                $requestData .= '<Request>';
                // Create Request Header
                $requestData .= '<Source>';
                // Add Requestor ID data
                $requestData .= '<RequestorID Client="' . $clientID . '" EMailAddress="' . $email . '" Password="' . $password . '" />';
                // Add Requestor Preferences data
                $requestData .= '<RequestorPreferences Language="' . $language . '" Currency="USD" Country="US" >';
                $requestData .= '<RequestMode>' . $requestMode . '</RequestMode>';
                $requestData .= '</RequestorPreferences>';
                $requestData .= '</Source>';
                // Create Request Body
                $requestData .= '<RequestDetails>
                                <SearchTransferPriceRequest>
                                <TransferPickUp>
                                <PickUpCityCode>LON</PickUpCityCode>
                                <PickUpCode>A</PickUpCode>
                                <PickUpPointCode>LCY</PickUpPointCode>
                                </TransferPickUp>
                                <TransferDropOff>
                                <DropOffCityCode>EDI</DropOffCityCode>
                                <DropOffCode>H</DropOffCode>
                                </TransferDropOff>
                                <TransferDate>
                                2013-11-11</TransferDate>
                                <NumberOfPassengers>2</NumberOfPassengers>
                                <PreferredLanguage>E</PreferredLanguage>
                                </SearchTransferPriceRequest>
                                </RequestDetails>';                    
//Save Request & Display XML errors
                libxml_use_internal_errors(true);
                $sxe = simplexml_load_string($requestData);
                if ($sxe === false) {
                    echo "Failed loading XML'n";
                    foreach(libxml_get_errors() as $error) {
                        echo "'t", $error->message;
                    }
                }

                $requestFile = simplexml_load_string($requestData);
                $requestFile->asXml('trequest.xml');