Running a script to check something before submitting a form. Can this be done?

Hi, I think I need to use PHP here which is why I'm posting to this ng. I have a form that upon the user pressing submit, calls a PHP form handler. What I'd like to do is upon the user pressing submit, I need to validate an email field within a MySQL database and only if it does NOT exist, call the form handler. If it exists, I'll juets return a custom page and NOT call the form handler. I'm wondering if I need some JavaScript here? If PHP is not involved, I apologize for the post. Don