Use phpstan return logic to infer return type from argument

[ildyria]

As migrating to php 8.2, I had to change utf8_encode (soon deprecated) to mb_convert_encoding.
Unfortunately this function when given a string as input returns a string, and when given an array returns an array.
This fix aims to leverage on phpdoc @return logic to infer the correct return type.

[ildyria]

IDK why this is failing, I don't think the file I modified has impact on this. :|

[shish]

Closing old PRs as lots has changed, feel free to re-open if this is still relevant

(FWIW I tested this in my current master branch, and it broke the generated code because it put ($string is string ? string : array)' verbatim into the function's return type)