Cloudflare does all requests to the origin using http/1.1 (see: https://support.cloudflare.com/hc/en-us/articles/214534978-Are-the-HTTP-2-or-SPDY-protocols-supported-between-Cloudflare-and-the-origin-server-) which is totally fine.
The problem however is that e.g. in PHP:
$_SERVER[‘SERVER_PROTOCOL’] will now return http/1.1 instead of http/2 even when the request is served with http2 by Cloudflare.
This matters for setting the status header, e.g. header( ‘http/1.1 404 Not found’ );
Are there any plans to “forward” the real protocol used?