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

$actual_link = "https://$_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

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

⚠️ This code has security implications because the client and the server can set HTTP_HOST and REQUEST_URI to arbitrary values. It is absolutely necessary to sanitize both values and do proper input validation (CWE-20). They must not be used in any security context.