This is a very basic test of the xmlhttp object.
Please view source to see how I did it, as I can't be bothered to document it... Which probably means that I will forget it in a few weeks time.
Very basic documentation: javascript is used to get the test.php file with a "?id=1" or "?id=2" parameter. That returns a string, which is split into an array, which is then filled into the select.
Go ahead, give it a spin and see how the select is changed without the page re-loading..