Dropdown menus affecting other dropdown menus...

I need some dropdown menus that automatically submits the chosen item. But the thing is:
I need to have one dropdown menu affect another dropdown menu...

I'll try and illustrate:
I am making a pricelist and have a Category, a Subcategory and a Manufacturer. I want three
dropdown menus, one for each Category, Subcategory etc.

When the page first loads all menus should be 'filled' with all available content, i.e. all categories,
all subcategories etc. But when a Category is chosen I want the other menus to be affected
by this, now only allowing appropriate content.

I would also like to have a Search box so you can search for key words...

At last I want a submit button that sends a request for a page with the values chosen...

I have PHP and MySQL on my server (but not ASP) and would preferably do it that way. I have found
a Javascript which should do soemthing similar but can't really get it to work...

Thanks in advance. Any help is appreciated .