Multiple multiple selects

Hi all, I have a question about handling several multiple select boxes. As we all know, doing multiple text boxes with the same name is easy: <input type="text" name="foo[]"> Handling one multiple select box is also easy: <select name="foo[]" multiple> But what about doing several multiple select boxes? <select name="foo[][]" multiple> doesn't seem to work. Can anybody give me some ideas? What I need for this is a 3-D array of $foo["select box"]["selected item"], and I can't seem to get that. Thanks!