Redirection based on submission for a gateway page PHP

So I have one php page that acts a like a gateway to entrance of the actual website based on a users response to question. e.g are you over 18? But my query is:

-how do I store a users answer to the question?

-how can perhaps a cookie or session variable record whether a user has confirmed or even visited the gateway page?

-how can I ensure that if the "actualpage.php" is cached in the users web browser it doesn't skip the gateway page if they haven't confirmed or visited the page?