<?php

$id = $_GET['id'];
$act = $_GET['act'];

switch($act) {
default :
case "display" : displayHTML(); break;
case "down" : down(); break;
}

function displayHTML($id) {
setcookie("visited", "true");

// print your HTML.
}

function down($id) {
if(! isset($_COOKIE['visited']) ) print "你盗联?";

$sql = "select path from TABLE where id=". $id;
//..............................
}
?>

<a href="dl.php?id=1&act=down">Download Now...</a>