Hi Barry,
As I said in my response, I used tools to check for the headers returned by the URL, the same information that is returned to search engine spiders.
There no way to know how the PHP script is made, for me, or for a search engine spider, as the PHP is executed server side and all you get back are the HTTP headers and content.
The url doesn't contain any id, it's simply the redirection url that is passed to it so the script probably look like this :
header('Location: '.$_GET['url'], true, 301);
?>
If the redirect.php file was receiving an ID instead of a direct URL, it would probably look more like this :
// stuff to connect to the database here
include('connect.php');
//a long line not well constructed that does a lot of stuff
$url = current(mysql_fetch_assoc(mysql_query("SELECT url FROM urls WHERE id=".(int)$_GET['id'])));
//redirect to the URL from the database
header('Location: '.$url, true, 301);
?>
It could be a way to trick user into thinking that the link is passing juice by doing a 301 redirect but blocking the file using robots.txt wich is not the case, there's also no Set-Cookie.
It is more likely to be a way to get statistics about what are the most popular links. For example, redirect.php probably add an entry to a database each time the url is reached to track the number of visitors sent to a particular website.
I hope my answer will help you understand the use of the redirect.php and the way it works. If you still have questions, do not hesitate to reply to my response.
Best regards,
Guillaume Voyer.