如何在PHP程序中防止盗链

example:

页面: dl.php
————————————————————————————–
<?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>