How to submit a form by selecting a drop-down menu item

[dainstructor]

I am looking for a way to submit my form without having the user click the submit button. I would like them to choose one option from a drop-down list, then I would dynamically populate the next drop-down list and so on, as in car buying websites. Choose Make --> then it populates the models ---> Choose Model.

I gave this a try in Javascript but was unsucessful. Can anyone help me do this in php? I prefer to do it in php if possible because the user may have Javascript disabled, but if anyway knows of a sure way of doing this in javascript, I'm also open to that as well.

Thanks in advance.

[cameraman]

php is server-side, so you can't do it there.
onchange="form.submit();"
has always worked for me.