How to find out if you’re using HTTPS without $_SERVER[‘HTTPS’]

I’ve seen many tutorials online that says you need to check $_SERVER['HTTPS'] if the server is connection is secured with HTTPS. My problem is that on some of the servers I use, $_SERVER['HTTPS'] is an undefined variable that results in an error. Is there another variable I can check that should always be defined?

Just to be clear, I am currently using this code to resolve if it is an HTTPS connection:

if(isset($_SERVER['HTTPS'])) {
    if ($_SERVER['HTTPS'] == "on") {
        $secure_connection = true;
    }
}