in src/str/utf8/select.php [27:33]
function slice(string $string, int $offset, ?int $length = null)[rx_local]: string {
invariant($length === null || $length >= 0, 'Expected non-negative length.');
$offset = _Private\validate_offset($offset, length($string));
/* HH_FIXME[2049] __PHPStdLib */
/* HH_FIXME[4107] __PHPStdLib */
return \mb_substr($string, $offset, $length, Encoding::UTF8);
}