PHP- Dependant dropdown based on Mysql queries

Hi All, I'm trying to generate a PHP page with 3 or 4 dropdown menus, each of one dependant upon each other, that means: #2 dropdown should change upon selecting something on dropdown #1, #3 should change based on the selection at #2 to finally submit the desired query. Each dropdown menu should be generated from a mysql query based on the previous selection. I need to generate mysql queries based on each dropdown selection and the page should reload the content for the next dropdown menu. I couldn't yet figure out how to accomplish this. Let me know if I make myself clear enough :) Any help is very welcome!! Thanks in advance, Francisco