unset()- newbie question

I can't seem to get unset() to work when I'm strying to delete an element from an array. I have a select list that is populated by an array, and i need to be able to delete items from the list by deleting them from the array, but I don't know how. I tried using unset($array['element']); but it doesn't work. Does anyone know why? Does anyone know of a way to use array_diff() to delete elements from an array? Thanks, Tom