Flash/PHP Email form

Hey all!
I basically made this exact form: http://www.kirupa.com/developer/actionscript/flash_php_email.htm but how can i make it so that the form will not submit untill all the fields have been filled out? If someone hasn’t filled out the Name, Email, and put something in the Message portion i dont want the “Send” button to do anything. Can someone help me out?
Thanks