listbox initiated sub listbox menus

I am working on a webpage being coded in php with a mysql backend.
Say for example the page has two listboxes COURSE and MODULES.

I am trying to get a page done where I require the user to select an item from COURSE listbox and by which the MODULES listbox gets populated with data relating to the COURSE selected .

I require possibly a link to information pertaining to how this can be achieved(most appreciated) or a possible brief explanation on how to achieve this.

THANKING YOU ONCE AGAIN TO HAVING LOOKED AT THIS...