PHP generates new rand() after submitting

I've got a problem that (I think) more people got. I generate a rand() in my PHP script, and if the form that I created submits, I don't want to create a new rand(), what's he actually doing right now.

I DON'T want to...

• Set the value in a SESSION or COOKIE
• Set the data in a hidden input

Because of.. Security reasons (it's a simple captcha)

Thanks.