Having Problem with AJAX / xmlHttp.responseXML

I am working on  a page that makes an ajax request to a php page, the php returns some xml.

like:

<?php
header("Content-type:text/xml");
header("Cache-Control: no-cache, must-revalidate");
//A date in the past
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

$q=$_GET["q"];

include_once("dbconnect.php");
$sql="SELECT * FROM user WHERE id = ".$q."";

$result = mysql_query($sql);

echo '<?xml version="1.0" encoding="ISO-8859-1"?>
<person>';
while($row = mysql_fetch_array($result))
 {
 echo "<firstname>" . $row['FirstName'] . "</firstname>";
 echo "<lastname>" . $row['LastName'] . "</lastname>";
 echo "<age>" . $row['Age'] . "</age>";
 echo "<hometown>" . $row['Hometown'] . "</hometown>";
 echo "<job>" . $row['Job'] . "</job>";
 }
echo "</person>";

?>

and  the stateChanged function of javascript code is

function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
alert(xmlHttp.responseText);
//here i am checking the responseText and it comes fine....

var xmlDoc=xmlHttp.responseXML.documentElement;
//Here responseXML not works. IE shows javascript error Object required

document.getElementById("firstname").innerHTML=xmlDoc.getElementsByTagName("firstname")[0].childNodes[0].nodeValue;

 document.getElementById("lastname").innerHTML=xmlDoc.getElementsByTagName("lastname")[0].childNodes[0].nodeValue;

}
}
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