I found the javascript to construct double linked comboboxes all over the web, they all had the same script.
My purpose was to fill them with data from a mysql database, so that it can be used for a form for entering entertainment purposes. The form needed to be able to update/edit the data.
There are three files included in the .zip file :
1- create the database and the two tables :
one for the main categories
one for the subcategories
They are linked together in the way Idnumber (main table) > vr_sleutel (second table)
2- conexxion.php wich contains your conexxion details
3- the file containing the code
4- a snapshot of how the code looks once executed
This will help you debug if something goes wrong
I suggest launching the code from within Dreamweaver
I use firefox wich has a helpfull javascript debugger as Jscript in case something is wrong does not throw an error but just does not work
info AT conexxion DOT be if there are any suggestions