Hierarchical menus?
i.e. You select a state, then a county in that state, then a city in that county?
There's two ways to do it. As you may already know (if not burn it in your memory) PHP is server side. All PHP code never gets processed by the browser and hence the user.
So you must communicate back with the server which option was selected in Menu or List A to give you the list that should appear in B.
There's two methods of doing this.
1. Forms. Submit the form and the value of option A to the server and produce a list for option B (the onChange() attribution of the <select> tag comes in handy for this.
2. AJAX. You submit a variable to a php page and it returns a list that can be ingested by JavaScript and used to populate list B.
Obviously, option 2 is harder but more preferred. The user does not have to wait for the page to reload (which might also reset the scroll position of the page to TOP on some browsers like IE)
Your question, by the way, has been answered before here on bytes:
http://bytes.com/topic/javascript/an...own-list-forms
Good luck,
Dan