Some of our software relies on the $_SERVER['HTTPS'] PHP variable to be set
to "on" when accessing pages via https://. So far I've found:

fastcgi_params HTTPS on;

but this turns it on for all pages, regardless of HTTP or HTTPS request
setting.

What am I missing?

When I add the HTTPS on parameter, our code works (i.e. detects HTTPS in
$_SERVER) but for every page, HTTP or HTTPs