dynamic drop down combo in php

i have a category drop down combo in my php page which shows categories like math and science. i want to show another subcategory combo box which will show subcategory of selected category, for example if user select math as category then subcategory combo should show subcategories of math like algebra, trigonometry and if user select science as category then subcategory combo should show subcategories of science like physics, biology, zoology etc. so i want this h开发者_C百科appen dynamically coz i am retrieving subcategory from database on the basis of selected category. how can i do this

You will have to use Ajax for that, see this php ajax tutorial.

http://www.w3schools.com/PHP/php_ajax_database.asp