You can accomplish this via pure PHP by submitting the form each time you have selected a drop-down value. That is not very nice and a bit of a hassle processing your form.
A much better solution is to use JavaScript and, better still, use Ajax. That way you can populate the boxes via an asynchronous call from the client to the server (php) program without refreshing your page or forcing the user to hit a 'submit' each time.
For Ajax, there is a demo (and the solution's code) at website dhtmlgoodies.com at page 'Ajax samples', which can be reached via link
Chained select boxes
Good luck.
Ronald :cool: