รบกวนช่วยด้วยค่ะ Error XML โดย PHP ค่ะ มันขึ้น error <b>Catchable fatal error</b>: Argument 1 passed to DOMNode::appendChild() must be an instance of DOMNode, null

<?php
function writeXML($xml)
{
	$fp = fopen('user.xml', 'w+');
	fwrite($fp,$xml);
	fclose($fp);
}

if(isset($submit))
{
		$name = $_POST['name'];
		$LastName = $_POST['lastname'];
		$position = $_POST['position'];
		$EN = $_POST['EN'];
		$dep =$_POST['dep'];
		$site=$_POST['site'];
		$tel = $_POST['tel'];
		$email = $_POST['email'];
		$username = $_POST['username'];
		$password = $_POST['password'];
		$time_regis = date("D d F Y h:i:s"); 
		
		

	$xmls = new DOMDocument('1.0','utf-8');
	if(@$load = $xmls->load("../user/user.xml"))
	{
		$i=0;
		$dom = new DOMDocument('1.0','utf-8');
		$nodeList = $xmls->getElementsbytagName('root')->item($i);
		$i++;
		
		$oldnode = $dom->importNode($nodeList, true);
		$dom->appendChild($oldnode);
		
		$xml_user = $dom->createElement( "User" );
		$xml_name = $dom->createElement( "Name",$name );
		$xml_lastname = $dom->createElement( "LastName",$lastname );
		$xml_position = $dom->createElement("Position",$position);
		$xml_en = $dom->createElement("Dep" ,$dep);
		$xml_site = $dom->createElement("site",$site);
		$xml_tel = $dom->createElement("tel",$tel);
		$xml_email = $dom->createElement("email",$email);
		$xml_username =$dom->createElement("password",$password);
		$xml_timeregis = $dom->createElement( "TimeUpFile", $timeregis );

		// Set the attributes.
		$xml_user->setAttribute( "ID", $i );
		//$count = xml.count("user",[user]);		
		

		// Append the whole bunch.
		$xml_user->appendChild( $xml_name );
		$xml_user->appendChild( $xml_lastname );
		$xml_user->appendChild( $xml_position);
		$xml_user->appendChild( $xml_en );
		$xml_user->appendChild( $xml_site );
		$xml_user->appendChild( $xml_tel);
		$xml_user->appendChild($xml_email );
		$xml_user->appendChild($xml_username );
		$xml_user->appendchild($xml_password );
		
	

		$dom->documentElement->appendChild($xml_user);


		// Parse the XML.
		$text = $dom->saveXML();
		print $text;

		writeXML($text);
		
	}
	else
	{
		// Set the content type to be XML, so that the browser will   recognise it as XML.
		header( "content-type: application/xml; charset=utf-8" );

		// "Create" the document.
		$xml = new DOMDocument( "1.0", "utf-8" );

		//Create root elemets
		$xml_root = $xml->createElement( "root" );
		// Create some elements.
		$xml_user = $xml->createElement( "User" );

		// Create another element, just to show you can add any (realistic to computer) number of sublevels.
			
		$xml_name = $xml->createElement( "Name",$name );
		$xml_lastname = $xml->createElement( "LastName",$lastname );
		$xml_position = $xml->createElement("Position",$position);
		$xml_en = $xml->createElement("Dep" ,$dep);
		$xml_site = $xml->createElement("site",$site);
		$xml_tel = $xml->createElement("tel",$tel);
		$xml_email = $xml->createElement("email",$email);
		$xml_username =$xml->createElement("password",$password);
		$xml_timeregis = $xml->createElement( "TimeUpFile", $timeregis );


		// Set the attributes.
		$xml_user->setAttribute( "ID", $i );
	

			// Append the whole bunch.
		$xml_user->appendChild($xml_name);
		$xml_user->appendChild($xml_lastname);
		$xml_user->appendChild($xml_position);
		$xml_user->appendChild($xml_en);
		$xml_user->appendChild($xml_site);
		$xml_user->appendChild($xml_tel);
		$xml_user->appendChild($xml_email);
		$xml_user->appendChild($xml_username);
		$xml_user->appendchild($xml_password);
		
		$xml_root->appendChild($xml_user);

		// Append the DOMXML.
		$xml->appendChild($xml_root);

		$text = $xml->saveXML();
		print $text;

		writeXML($text);
		
	}
}
else
{
?>
<b>Add</b><p/>
<form method="post" name="formxml" action="<?=$SERVER_['PHP_SELF'] ?>">
<table width="769" border="0" cellspacing="0">
<tr><td height="33" colspan="4"><center>
   Set  Track Element 
 </center></td></tr>
<tr>
  <td width="124">Name : </td><td width="291"><input type="text" name="name"></td>
  <td width="88">LastName :</td>
  <td width="258"><label>
    <input type="text" name="lastname" id="lastname">
  </label></td>
</tr>

<tr>
  <td>Position : </td><td><input type="text" name="position" /></td>
  <td>EN : </td>
  <td><label>
    <input type="text" name="en" id="en">
  </label></td>
</tr>
<tr>
  <td>Department :</td><td><input type="text" name="dep" /></td>
  <td>Site :</td>
  <td><label>
    <input type="text" name="site" id="site">
  </label></td>
</tr>
<tr><td colspan="4"></td></tr>
<tr>
  <td>Tel :</td>
  <td><label>
    <input type="text" name="tel" id="tel">
  </label></td>
  <td>Email :</td>
  <td><label>
    <input type="text" name="email" id="email">
  </label></td>
</tr>
<tr>
  <td>User Name :</td>
  <td><label>
    <input type="text" name="textfield" id="textfield">
  </label></td>
  <td>Password :</td>
  <td><label>
    <input type="text" name="textfield2" id="textfield2">
  </label></td>
</tr>
<tr><td colspan="4"><center><input type="submit" name="submit" value="  Add  ">
  <label>
    <input type="reset" name="Reset" id="button" value="Reset">
  </label>
</center></td></tr>
</table>
</form>
<?
}
?>