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

This should always work even when $_SERVER['HTTPS'] is undefined:

function isSecure() {
  return
    (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off')
    || $_SERVER['SERVER_PORT'] == 443;
}

The code is compatible with IIS.

From the PHP.net documentation and user comments :

1) Set to a non-empty value if the script was queried through the HTTPS protocol.

2) Note that when using ISAPI with IIS, the value will be "off" if the request was not made through the HTTPS protocol. (Same behaviour has been reported for IIS7 running PHP as a Fast-CGI application).

Also, Apache 1.x servers (and broken installations) might not have $_SERVER['HTTPS'] defined even if connecting securely. Although not guaranteed, connections on port 443 are, by convention, likely using secure sockets, hence the additional port check.

Additional note: if there is a load balancer between the client and your server, this code doesn't test the connection between the client and the load balancer, but the connection between the load balancer and your server. To test the former connection, you would have to test using the HTTP_X_FORWARDED_PROTO header, but it's much more complex to do; see latest comments below this answer.

My solution (because the standard conditions [$_SERVER['HTTPS'] == 'on'] do not work on servers behind a load balancer) is:

$isSecure = false;
if (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') {
    $isSecure = true;
}
elseif (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' || !empty($_SERVER['HTTP_X_FORWARDED_SSL']) && $_SERVER['HTTP_X_FORWARDED_SSL'] == 'on') {
    $isSecure = true;
}
$REQUEST_PROTOCOL = $isSecure ? 'https' : 'http';

HTTP_X_FORWARDED_PROTO: a de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS http://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Common_non-standard_request_headers

Chacha, per the PHP documentation: "Set to a non-empty value if the script was queried through the HTTPS protocol." So your if statement there will return false in many cases where HTTPS is indeed on. You'll want to verify that $_SERVER['HTTPS'] exists and is non-empty. In cases where HTTPS is not set correctly for a given server, you can try checking if $_SERVER['SERVER_PORT'] == 443.

But note that some servers will also set $_SERVER['HTTPS'] to a non-empty value, so be sure to check this variable also.

Reference: Documentation for $_SERVER and $HTTP_SERVER_VARS [deprecated]