Chained Select Boxes using PHP / MySQL / AJAX / jQuery http://www.blueicestudios.com/chained-select-boxes-using-php-mysql-ajax/

Dynamic Dependent Select Box using Jquery and Ajax http://www.9lessons.info/2010/08/dynamic-dependent-select-box-using.html

Dynamic drop down(SELECT TAG) PHP AJAX https://www.youtube.com/watch?v=GVHphoGi5dY