Here is a great example of cascading dropdowns that is populating state dropdown on change of country dropdown.Its very easy to integrate, light weight, platform independent doesn’t matter your application is in php or ASP.net or other,it will work as it is using core Javascript.
Just include javascript file as:

js_countries

And put following html code in Body section:

And that’s it, you will see two dropdowns on your page, one of country and other one is of states;

Example:

LIVE DEMO

DOWNLOAD CODE:

–Populate State Dropdown onChange of Country Dropdown using Javascript–