php,xml,ups shipping: "Cannot view XML input using XSL..." Where's the bug?

so I'm deploying a shipping calculator using this function, and I'm getting this error only when I use IE. Firefox does just fine with the shipping calculator. I'm unfamiliar with this error, and google searches tell me that it's a problem with the xml format. Problem is: the XML response from UPS's shipping calculation server shouldn't have this content. So, my question is: where do you think the bug is? I appreciate answers and suggestions of where to check. I'm drawing a blank.

Error when using IE (vrs 8. 7, and 6):

The XML page cannot be displayed Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.

Incorrect syntax was used in a comment. Error processing resource 'http://mgxvideo.com/mgxcopy-alpha-3/shopping/cart_displa...

----------^

Portion of the php code requesting from the server:

$ch = curl_init("https://www.ups.com/ups.app/xml/Rate");
        curl_setopt($ch, CURLOPT_HEADER, 1);
        curl_setopt($ch,CURLOPT_POST,1);
        curl_setopt($ch,CURLOPT_TIMEOUT, 90);
        curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
        curl_setopt ($ch, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt ($ch, CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($ch,CURLOPT_POSTFIELDS,$data);
        $result=curl_exec ($ch);
    echo '<!-- '. $result. ' -->'; // THIS LINE IS FOR DEBUG PURPOSES ONLY-IT WILL SHOW IN HTML COMMENTS
        $data = strstr($result, '<?');
        $xml_parser = xml_parser_create();
        xml_parse_into_struct($xml_parser, $data, $vals, $index);
        xml_parser_free($xml_parser);
        $params = array();
        $level = array();
        foreach ($vals as $xml_elem) {
         if ($xml_elem['type'] == 'open') {
        if (array_key_exists('attributes',$xml_elem)) {
             list($level[$xml_elem['level']],$extra) = array_values($xml_elem['attributes']);
        } else {
             $level[$xml_elem['level']] = $xml_elem['tag'];
        }
         }
         if ($xml_elem['type'] == 'complete') {
        $start_level = 1;
        $php_stmt = '$params';
        while($start_level < $xml_elem['level']) {
             $php_stmt .= '[$level['.$start_level.']]';
             $start_level++;
        }
        $php_stmt .= '[$xml_elem[\'tag\']] = $xml_elem[\'value\'];';
        eval($php_stmt);
         }
        }
        curl_close($ch);
        #print_r($params);
        #echo "<br/><br/>";
        return $params['RATINGSERVICESELECTIONRESPONSE']['RATEDSHIPMENT']['TOTALCHARGES']['MONETARYVALUE'];

And here's what Firefox echoes as the result from the xml request (in the above code, it's the line that says "THIS LINE IS FOR DEBUG PURPOSES ONLY":

<!-- HTTP/1.1 100 Continue

HTTP/1.1 200 OK
Date: Fri, 26 Jun 2009 21:58:04 GMT
Server: Apache
Pragma: no-cache
Content-Length: 1524
Content-Type: application/xml

<?xml version="1.0"?><RatingServiceSelectionResponse><Response><TransactionReference><CustomerContext>Bare Bones Rate Request</CustomerContext><XpciVersion>1.0</XpciVersion></TransactionReference><ResponseStatusCode>1</ResponseStatusCode><ResponseStatusDescription>Success</ResponseStatusDescription></Response><RatedShipment><Service><Code>02</Code></Service><RatedShipmentWarning>Your invoice may vary from the displayed reference rates</RatedShipmentWarning><BillingWeight><UnitOfMeasurement><Code>LBS</Code></UnitOfMeasurement><Weight>6.0</Weight></BillingWeight><TransportationCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>14.34</MonetaryValue></TransportationCharges><ServiceOptionsCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>0.00</MonetaryValue></ServiceOptionsCharges><TotalCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>14.34</MonetaryValue></TotalCharges><GuaranteedDaysToDelivery>2</GuaranteedDaysToDelivery><ScheduledDeliveryTime></ScheduledDeliveryTime><RatedPackage><TransportationCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>14.34</MonetaryValue></TransportationCharges><ServiceOptionsCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>0.00</MonetaryValue></ServiceOptionsCharges><TotalCharges><CurrencyCode>USD</CurrencyCode><MonetaryValue>14.34</MonetaryValue></TotalCharges><Weight>6.0</Weight><BillingWeight><UnitOfMeasurement><Code>LBS</Code></UnitOfMeasurement><Weight>6.0</Weight></BillingWeight></RatedPackage></RatedShipment></RatingServiceSelectionResponse> -->

Ideas?

Solution

Technically, you can't have '--' inside of XML comments. So you should change:

<!------------------- main content ------------------------->

<!--                  main content                     -->

...or something similar. If UPS is sending it. You can replace it before you forward it to the browser.

EDIT

Regarding displaying the markup rather than rendered HTML: If you're seeing the same thing as in Firefox (<RatingServiceSelectionResponse, etc.) that's not HTML--it's XML. You'll either want to transform it with XSLT or use XPath (or XQuery, etc.) to fetch particular values out of it. Looks like you could also use the $myRate->getRate() function from ups-php API you referenced.