>This is a setting that affects the application globally;

nope. it's for `UTF8::clean`

if i replace `mb_substitute_character('none')` with `mb_substitute_character('entity')` in `bootstrap.php`, `UTF8::clean` will not work as it claim.
>* Recursively cleans arrays, objects, and strings. Removes ASCII control
>* codes and converts to the requested charset while silently discarding
>* incompatible characters.