MSXML2.DOMDocument Object required error if file extension is not .xml - php

Question

Working on an application to retreive shipping rates as XML from a web service. The web service is PHP based. The client is VB Script, specifically HTA.
During initial development I used an XML file on the server as a test. Here is the content of that file:
<shipping>
<rate>
<method>FEDEX GROUND</method>
<description>FedEx Ground</description>
<amount>10.00</amount>
</rate>
<rate>
<method>FEDEX 2 DAY</method>
<description>FedEx 2nd Day</description>
<amount>20.00</amount>
</rate>
<rate>
<method>FEDEX NEXT DAY</method>
<description>FedEx Next Day</description>
<amount>30.00</amount>
</rate>
</shipping>
When I use MSXML2.ServerXMLHTTP to get that file I have no issues using MSXML2.DOMDocument to parse the XML.
If I attempt to get the exact same XML from a PHP script, or even if I copy the XML file to a filename with a non-xml extension, like .txt, I get an Object Required error trying to access the document elements.
When getting the data from PHP I am setting the header like so:
header('Content-Type: application/xml; charset=utf-8');
Here is the VB Script code:
Set xmlhttp = CreateObject("MSXML2.ServerXMLHTTP")
xmlhttp.open "GET", "http://craig.clearos.lan/c9pets/api/shippingRates", False
xmlhttp.send
results=xmlhttp.ResponseText
Set xmlDoc = CreateObject("MSXML2.DOMDocument")
xmlDoc.load(xmlhttp.responseXML)
Set ratesNodeCollection = xmlDoc.DocumentElement.getElementsByTagName("rate")
This errors out on the last line with "Object required: 'xmlDoc.DocumentElement'"
If I change line 2 to this
'xmlhttp.open "GET", "http://craig.clearos.lan/sage/hello.xml", False
it works as expected.
Again, I have verified that the PHP code and the .txt file contain the exact same XML as the .xml file.
I have added parseError trapping and I'm getting the message "XML document must have a top level element." This makes no sense to me. There is a top level element named I've also added to the .xml file and the output of PHP. This did not change anything. I tried setting the header to text/xml instead of application/xml, which also did nothing.
For reference here is the very simple php code that builds the XML.
$shipping=array(0=>array('method'=>'FEDEX GROUND', 'description'=>'FedEx Ground', 'amount'=>'10.00'),
1=>array('method'=>'FEDEX 2 DAY', 'description'=>'FedEx 2nd Day', 'amount'=>'20.00'),
2=>array('method'=>'FEDEX NEXT DAY', 'description'=>'FedEx Next Day', 'amount'=>'30.00'));
$xml=new SimpleXMLElement('<shipping/>');
foreach($shipping as $row){
$rate=$xml->addChild('rate');
$rate->addChild('method', $row['method']);
$rate->addChild('description', $row['description']);
$rate->addChild('amount', $row['amount']);
}
Header('Content-type: application/xml');
print($retVal->asXML());