将字符串按照指定长度分割

public function str_split_unicode($str, $l = 0)
    {
        if ($l > 0) {
            $ret = array();
            $len = mb_strlen($str, "UTF-8");
            for ($i = 0; $i < $len; $i += $l) {
                $ret[] = mb_substr($str, $i, $l, "UTF-8");
            }
            return $ret;
        }
        return preg_split("//u", $str, -1, PREG_SPLIT_NO_EMPTY);
    }