Is there documentation showing how to force HTTPS on Google App Engine - Flexible PHP?
I tried placing secure on app.yaml and I had no luck. 
I also tried in the nginx-app.conf placing this but no luck.
I also tried
set $test "";
if ($http_x_forwarded_proto = 'http') {
    set $test "http";
}
if ($test = 'http-non-cron') {
    return 301 https://$host$request_uri;
}
Other Option I tried
<?php     
     header("Strict-Transport-Security: max-age=31536000; includeSubDomains; preload");  
     exit();  
?>
but no luck.
 
    