<?php
$yoursite = "google.com"; 
$yoursite2 = "www.google.com";

$referer = $_SERVER['HTTP_REFERER'];

//Check if browser sends referrer url or not
if ($referer == "") { //If not, set referrer as your domain
$domain = $yoursite;
} else {
$domain = parse_url($referer); //If yes, parse referrer
}

if($domain['host'] == $yoursite || $domain['host'] == $yoursite2) {

//Run your dowloading code here normally


} else {

//The referrer is not your site, we redirect to your home page
header("Location: http://siten.com/kontrolsayfasi.php");
exit(); //Stop running the script

}

?>