Validate form elements on checkout page

Hey Ray!

A couple of years ago I was using coldfusion, and had this question and during a mentor session, and you showed me a quick way to validate some of my form items before bad info can be passed from the checkout page to the confirm page.

If bad info was entered a small dialog box popped up and would not allow the page to be submitted.....so if for instance an email address was not valid or if a cvv code was not entered or if the cc number was not the correct amount of digits.

Now I have moved to PHP and it has been a while and I cannot remember how we did it. I think it was some type of form action but I don't even know where to start.

Any ideas?

Thanks

Grant