Stuff

<?php

$someValue = "Hey look at this!";

if (isset($_COOKIE["theCookie"])) { // If the cookie is set
        $theValue = $_COOKIE["theCookie"];
        echo $theValue;
        // Other actions
} else {
        setcookie("theCookie", $someValue, time()+3600); // Set for one hour, change as needed
        header("Location: http://www.trinity-ro.com/files/popup.htm"); // Make sure full address is set
        exit; // Not that important
}

?>

<html>
Other stuff goes here, but remember if cookie isn't set it won't get output.