Sending variable to php page - login page

I’m trying to login to a password protected php page through a form in flash.
When the user provides correct information the flashmovie opens a php-page, but how do I protect the php page from unautorized accesses?

Can I send a variable containing the user/pass given?
I know I could tell flash to “getURL www.address.com/admin.php?user=user&pass=12345”, but then everyone who checks history can access the page without even having to access the flash page…

Anyone getting what I mean?

Thanks!!
Regards,
Michael