In a form I have a dropdown field result of an application. I have a combo second field whose content will depend on the choices made in the first box.
As not get the value of the initial field (without sending the form to submit server).
onChange = Submit (). In <form you put action="your_page_php.php"
In your_page_php you check that $ _POST ['name_list1']!="" set your database and load your second list.
For the selected value of your first list, you have to put in all:
Do a Google search on "Ajax-related lists ".
Thanks to Alain42 for this tip.