refreshing form select boxes with javascript. possible?

Hi, I have a MySQL database that contains a table of composer names/ID numbers, another of symphonies/ID numbers, and another of symphonies/composer ID's. The problem is that under the search form now, the search by name does a simple query to the symphonies/ID table and gets back a result of all available symphonies. This is cumbersome. What I would like to be able to do, is once the composer name select box on the search form is selected, have the symphony name form re-update itself to include only those symphonies done by the composer. I imagine this would use a javascript "onSelect," but every time I try to include PHP code in a onselect it gets run anyway. Any way around this? Can javascript call an exterior PHP include or something? Without refreshing the whole page? Any ideas? the url for the project is symphonynet.datablocks.net. It's the free datablocks php/MySQL deal, sometimes the connection is pretty slow. Thanks, Kepa Lyman