On Screening Page:
=========================================================
<a href="accept.php">Click here</a> if you accept our terms and conditions.
Accept.php:
=========================================================
<?php
session_start();
$_SESSION['accepted']=true;
//redirects people to a 2nd index page
header("Location: main.php");
?>
On all other pages:
==========================================================
<?php
session_start();
if(!isset($_SESSION['accepted'])) header("Location: screeningpage.php");
?>