This does not parse. You mean
deb https://packages.sury.org/php/ stretch main
Then, when I follow your manual, for debian 9, the vhost for froxlor gives a 404 not found where you want me to set database name etc. So after this:
Froxlor doesn't load, even if the folder exists at the location you described. Probably because there's no php fpm config whatsoever at that stage? Anyway, you need to change the order in your how to. Because now I'm pretty much stuck, not knowing where to continue..