I'm honestly not sure what you're trying to do with the header, it's not any Ajax method I've ever been taught. The header method you're doing looks just a few lines short of outputting the XML to a download prompt.
Here's my favorite way to do AJAX. Simple, understandable, and quick.
- Include Jquery.
- Send via Jquery Ajax to a separate processing page. The page will receive the data you setup as a $_REQUEST variable, with the method depending on whether you defined it as a POST or not (defaults to a GET)
- The processing page --does-- stuff with the REQUEST data and may or may not respond back to the page. This is where you can do stuff like update divs, alert that it worked, etc.
Here's a great tutorial. Focus on the code under "Hello Ajax, Meet Jquery"
If you get yourself any more of those mushrooms, a PHP familiar way to do AJAX is with XAJAX. It allows you to do asynchronous calls to PHP functions. Be aware, though, that the forums are not the most english-friendly and documentation is a bit cryptic.