How to get the full URL in PHP

$actual_link = "https://$_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]";

⚠️ Using this code has security implications because the client can set HTTP_HOST and REQUEST_URI to arbitrary values. It is absolutely necessary to sanitize both values and do meaningful input validation.