I'm having a problem with my responsehandler.php file. I get the following error.

Integration Issue Detail

Time of occurrence: Sep 8, 2007 4:23:05 AM PDT Error: Error parsing XML; message from parser is: Content is not allowed in prolog. Warnings: XML We Received: Sat Sep 8 7:23:05 EDT 2007:- Invalid
XML We Sent: <?xml version="1.0" encoding="UTF-8"?>
<merchant-calculation-callback xmlns="http://checkout.google.com/schema/2" serial-number="changed for display">
<buyer-id>changed for display</buyer-id>
<shopping-cart>
<items>
<item>
<tax-table-selector>Taxable Goods</tax-table-selector>
<merchant-item-id>209</merchant-item-id>
<item-name>Window Alert Hawk Decals</item-name>
<item-description></item-description>
<unit-price currency="USD">6.99</unit-price>
<quantity>1</quantity>
<merchant-private-item-data>changed for display=</merchant-private-item-data>
</item>
</items>
<merchant-private-data>


<session-data>changed for display;zenid</session-data>


<product-data>;209</product-data>


<ip-address>changed for display</ip-address>


</merchant-private-data>
</shopping-cart>
<calculate>
<addresses>
<anonymous-address id="changed for display">
<city>Albany</city>
<country-code>US</country-code>
<region>NY</region>
<postal-code>12210</postal-code>
</anonymous-address>
</addresses>
<merchant-code-strings />
<shipping>
<method name="UPS: Ground" />
</shipping>
<tax>false</tax>
</calculate>
<buyer-language>en_US</buyer-language>
</merchant-calculation-callback>