Need PHP help: Get error message when user has blank forms

Hello.

I’m working with this great PHP form-mail script, found here at Kirupa:

http://www.kirupa.com/web/form_mailer.htm

could someone please tell me what I could add to make this script give an error message to a user who doesn’t enter specific fields? For example, if they do not enter a name or e-mail address it tells them “Error, please fill out all fields” and redirects.

thanks!!

-MD