[php] form is working but allow to submit blank

hello
I’m using this form mail: http://www.kirupa.com/developer/actionscript/flash_php_email.htm

Everything is working perfect but i wanted to prevent ppl to submit when they didn’t fill the form…i’m receiving a lot of mails daily empty because ppl click on the send button without write nothing. what i have to do?

Thanks in advance for any help