I generally like the idea, but it seems many (most?) real-world implementations actually use mb_strpos() !== false by default.
https://github.com/danielstjules/Stringy/blob/df24ab62d2d8213bbbe88cc36fc35a4503b4bd7e/src/Stringy.php#L206-L215 https://github.com/illuminate/support/blob/6eff6cff19f7ad5540b9a61a9fb3612ca8218c19/Str.php#L157-L166 So there should definitely be an mb_str_contains in ext/mbstring in addition to the regular str_contains proposed here.
