responseXML is always null on php script return

I've been fighting with this for a while now.  I have searched the web and still could not find a solution to my problem.

I have JavaScript that calls a server PHP script synchronously using the XMLHttpRequest object.  I'm returning a simple XML document from the server, but the responseXML property is always null and the Content-Type header is always text/html.  Please take a look at my code and let me know what it is I'm doing worng.

I tested the PHP script on both FireFox and IE and they display the XML document and using FireFox web developer add-on I can see that the header is set to text/xml.

Thank you in advance for your help.

///////////////////////////////////////////////////////////////////////////////
JavaScript:
var xmlhr = false;
if(typeof XMLHttpRequest != "undefined")
	xmlhr = new XMLHttpRequest();
else if(typeof ActiveXObject != "undefined"){
	try{
		xmlhr = new ActiveXObject("MSXML2.XMLHTTP.3.0");
	}
	catch(e){
		alert("Error creating XMLHttpRequest: " + (e.description ? e.description : e.message));
	}
}
 
if(xmlhr){
		// Open the connection to the server
		xmlhr.open("POST", script.php, false);
		xmlhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded")
		xmlhr.send(postdata);
		if(xmlhr.status == HttpStatus.OK){
			// This always displays "text/html"
			alert(xmlhr.getResponseHeader("Content-Type"));
			var value = xmlhr.responseXML;
			// This always displays null
			alert(value);
		}
}
///////////////////////////////////////////////////////////////////////////////
PHP Script:
<?php
header('Content-Type: text/xml');
header("Cache-Control: no-cache, must-revalidate");
echo(DisplayError("There was an error connecting to the server \"No Function Selected\".  Please try agin later."));
 
///////////////////////////////////////////////////////////////////////////////
// This function will return the specified error message to the caller
//
function DisplayError($message)
{
	$xmlText = "<?xml version=\"1.0\" encoding=\"utf-8\"?>";
	$xmlText .= "\n<response>";
	$xmlText .= "\n\t<error>$message</error>";
	$xmlText .= "\n</response>";
	
	return $xmlText;
}
///////////////////////////////////////////////////////////////////////////////
?>
///////////////////////////////////////////////////////////////////////////////

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