Have a look at $_SERVER['REQUEST_URI'], i.e.

$actual_link = "http://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";

(Note that the double quoted string syntax is perfectly correct)

If you want to support both HTTP and HTTPS, you can use

$actual_link = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? "https" : "http") . "://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";

Editor's note: using this code has security implications. The client can set HTTPHOST and REQUESTURI to any arbitrary value it wants.