fix: explode with empty separator in exemplar

[vnkmpf]

Explode with empty separator errs: https://3v4l.org/RevTj
Use mb_str_split instead: https://3v4l.org/VAKbR

[neenjaw]

I need a bit more of an explanation. Why does CI pass using the exemplar right now:? CI Logs

[vnkmpf]

Maybe I didn't express myself clearly, why I think it's a problem.

The exercise wants you to provide types, so the tests don't test the actual behavior, which is provided in the file, but only tests correct types are added.

Why I think it should be fixed is that it shows you a code that would not work. As seen in 3v4l examples.
Taken to a bit extreme example - this code would still pass tests, but doesn't "feel right".

function letters(string $word): array
{
    throw new \RuntimeException();
}

[neenjaw]

Makes sense