Dropdownlist() Value

Hi,
I need to know when an option is selected in dropdownlist its corresponding value id need to be stored in database.
I have two tables.
Department
dept_id
dept_name
Access
acl_id
dept_id

I did changes such as the department names are displayed in Access _form.php. When I'm selecting department name from dropdownlist.It shows "Department cannot be blank".I need to store that corresponding department id value in Access table.

Can anybody help over??
Thanks in advance