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");
?>