i am using HTML_AJAX pear class for ajax related functions.I would like to implement listbox population (using mysql) using HTML_AJAX Pear class

i am not getting an idea how to do this?i tried to get the answer from PEAR.But ,i am not able to do.If anyone done this using HTML_AJAX,please help me and what r the functions i should use for doing list box population (list box population means-clicking in the one selected select box will populate the next select box available in the page based on the first one -ie chained select box)