At about line 129,it is:
return mb_strrpos($str, $search);
and it should be:
return mb_strrpos($str, $needle, $offset);

in function utf8_strpos
if ($offset === false)
should be:
if (is_null($offset))

in function utf8_substr
if ($length === false)
should be:
if (is_null($length))

the are on line 141、177、211