For Apache server:

• 1: Add in htaccess file

<IfModule mod_env.c>
    SetEnv CI_ENV production
</IfModule>

between ### InvoicePlane htaccess and <IfModule mod_rewrite.c> or at end.

• 2: Rename htaccess to .htaccess

That's all.

For Nginx:

• 1: Add inside vhosts configuration:

  server {
        location ~ \.php$ {
            fastcgi_param CI_ENV production;
        }

• 2: Save and run nginx syntax check (just to avoid you from cursing me):

nginx -t

Notes: 3 modes are possible: production, development or testing

Src
Found by this Search