Populating A Combo Box From Another Combo Box?

I am using javascript to populate a number of text boxes based on the data item selected in the 1st combo box.

I am now working on making the selection of that combo box populate another combo box with items from another table. I can hard the sql code for the 2nd combo box so that when I select an option it populates more text boxes, but want the contents of the 2nd combo box to dynamically change depending on the selection of the 1st box. I would like this to do it without refreshing the page after each 1st combo box selection.

The 2nd box does not populate with the below code as I guess the variable isn't populated, I can put '1' in and it selects products with that prodid, but as explained above i'd like it to read the prodid from the 1st combo box.

Code:
<?php
require "session_logincheck.php";
function selectProductAndPopulate()
{

[Code].....