Well, I'm sorry, but my script doesnt do that, but what you discribed doesnt seem to different of difficult, the only thing is that the user is going to have to refresh the page every time he makes a selection unless you use java scripts of some sort. But here is how you can do it:
use the previous script to make your pull down menu, set the value of the options to the catagory identification number/marker, when the page reloads, you can then use that catagory ID to select the options for that catagory, you can do that in your query:
mysql_query("select * from catagory_options where cat_id = $catagory");
and then run the previous script again, substituting that first query for this one, except that return value will be a diffent thing, like a page number, or catagory number, at which point when the page reloads again, you can display the appropriate page.
hope it helps, let me know if I can help any more