public function insert($substring, $index)
{
$stringy = static::create($this->str, $this->encoding);
if ($index > $stringy->length()) {
return $stringy;
}
$start = \mb_substr($stringy->str, 0, $index, $stringy->encoding);
$end = \mb_substr($stringy->str, $index, $stringy->length(), $stringy->encoding);
$stringy->str = $start . $substring . $end;
return $stringy;
}