如何将新数据添加到XML

register.htm

<HTML XMLns="http://www.w3.org/1999/xHTML">
<head>      
    <link rel="stylesheet" type="text/css" href="mystyle.css" />
    <script type="text/javascript" src="shoponline.js"></script> 
    <title>ShopOnline Register Page</title>
</head>

<body>
    <H3>ShopOnline</H3>     

    <table width="100%">
        <tr>
            <td><button class="button" name="btnHome" value="Home" onclick="location.href='login.htm';">Home</button></td>
            <td><button class="button" name="btnListing" value="Listing" onclick="location.href='listing.htm';">Listing</button></td>
            <td><button class="button" name="btnBidding" value="Bidding" onclick="location.href='bidding.htm';">Bidding</button></td>
            <td><button class="button" name="btnMaintenance" value="Maintenance" onclick="location.href='maintenance.htm';">Maintenance</button></td>
            <td><button class="button" name="btnLogout" value="Logout" onclick="location.href='#';">Log Out</button></td>
        </tr>
    </table>

    <hr><br><br>

    <I>To register a new account with ShipOnline, please complete the registration form below.</I>

    <br><br>

    <form>
        <div class="form">
            <fieldset>
                <legend>Registration Details</legend>
                <p style="text-align:left;">* Required Fields</p>
                <br>
                <table>
                    <tr>
                        <td style="text-align:right;">First Name <label>*</label></td>
                        <td><input type="text" id="txtFName" name="txtFName" class="textbox" placeholder="Enter Your First Name"
                            oninvalid="this.setCustomValidity('Please Enter Your First Name!')" oninput="setCustomValidity('')" required /></td>
                    </tr>
                    <tr>
                        <td style="text-align:right;">Surname <label>*</label></td>
                        <td><input type="text" id="txtSurname" name="txtSurname" class="textbox" placeholder="Enter Your Surname"
                            oninvalid="this.setCustomValidity('Please Enter Your Surname!')" oninput="setCustomValidity('')" required /></td>
                    </tr>
                    <tr>
                        <td style="text-align:right;">Email <label>*</label></td>
                        <td><input type="email" id="txtEmail" name="txtEmail" class="textbox" placeholder="Enter Your Email Address"
                            oninvalid="this.setCustomValidity('Please Enter Your Email!')" oninput="setCustomValidity('')" required /></td>
                    </tr>
                    <tr>
                        <td></td>
                        <td>
                            <input type="button" name="btnRegister" value="Register" onclick="register()" />
                            <input type="button" name="btnReset" value="Reset" onclick="reset('registerForm')" />
                        </td>
                    </tr>
                </table>
            </fieldset>
        </div>  
    <form>

    <div id="information"></div>

</body>
</HTML>

shoponline.js

var xHRObject = false;

if (window.XMLHttpRequest)
    xHRObject = new XMLHttpRequest();
else if (window.ActiveXObject)
    xHRObject = new ActiveXObject("Microsoft.XMLHTTP");


function reset(formID) {
    switch (formID) {
        case 'loginForm':
            document.getElementById('txtEmail').value = "";
            document.getElementById('txtPassword').value = "";
            break;
        case 'registerForm':
            document.getElementById('txtFName').value = "";
            document.getElementById('txtSurname').value = "";
            document.getElementById('txtEmail').value = "";
            break;
    }
}

function login() {
    var email = document.getElementById('txtEmail').value;
    var pass = document.getElementById('txtPassword').value;

    xHRObject.open("GET", "login.php?id=" + Number(new Date) + "&email=" + email + "&pass=" + pass, false); 
    xHRObject.send(); 

    if (xHRObject.responseText == true)
        document.location.href = "bidding.htm";
    else
        window.alert("Login Failure!");
}

function register() {
    var isFound = false;
    var email = document.getElementById('txtEmail').value;
    var firstName = document.getElementById('txtFName').value;
    var surName = document.getElementById('txtSurname').value;

    xHRObject.open("GET", "register.php?id=" + Number(new Date) + "&email=" + email + "&firstName=" + firstName + "&surName=" + surName, false);
    xHRObject.send();

if (xHRObject.responseText == true)
    window.alert("Registration Failure!");
else
    window.alert("Registration Success!");
}

customer.xml

<customers>
<customer>
    <ID>C1</ID>
    <FirstName>Jack</FirstName>
    <SurName>Wong</SurName>
    <Email>jack@hotmail.com</Email>
    <Password>081292</Password> 
</customer> 

<customer>
    <ID>C2</ID>
    <FirstName>Ashley</FirstName>
    <SurName>Rachael</SurName>
    <Email>ashley@hotmail.com</Email>
    <Password>081292</Password> 
</customer>

<customer>
    <ID>C3</ID>
    <FirstName>Vongola</FirstName>
    <SurName>Steve</SurName>
    <Email>vongola@hotmail.com</Email>
    <Password>081292</Password> 
</customer>
</customers>

register.php

<?php
$xmlFile = "customer.xml";
$isFound = false;
$count = 0;

// Check the XML is exist or not. 
// If does, do checking against the email address.
if (file_exists($xmlFile)) {
    $dom = new DOMDocument();
    $dom->load($xmlFile);
    $customer = $dom->getElementsByTagName("customer"); 

    foreach($customer as $node) { 
        $fName = $node->getElementsByTagName("FirstName");
        $valueFName = $fName->item(0)->nodeValue;

        $email = $node->getElementsByTagName("Email");
        $valueEmail = $email->item(0)->nodeValue;   

        if($_GET["email"] == $valueEmail)
            $GLOBALS["isFound"] = true;

        $GLOBALS["count"]++;
    }

    if ($isFound != true) {
        // UNIQUE EMAIL, START UPDATE XML & SEND MAIL
        $firstName = $_GET["firstName"];
        $surName = $_GET["surName"];
        $email = $_GET["email"];
        $password = random_password(8);

        $xml = new SimpleXMLElement($xmlFile);

        $customer = $xml->addChild("customer");
        $customer->addChild("ID", "C" .($count + 1));
        $customer->addChild("FirstName", $firstName);
        $customer->addChild("SurName", $surName);
        $customer->addChild("Email", $email);
        $customer->addChild("Password", $password);

        $xml->saveXML($xmlFile);
    }

} else {
    // XML FILES NOT EXIST. STRAIGHT AWAY CREATE A NEW ONE AND SEND EMAIL
}

ECHO ($isFound);

?>

Basically, I was used AJAX for server and client communication. Therefore, when end-users finish input their information at register.htm and it will go to javascript file using XMLHttpsRequest.open to register.php for updating data on XML. But it doesn't update my XML data. Any advise will be appreciate.