public static function chars($str)
    {
        if (self::$use_mbstring) {
            $chars = [];
            for($i = 0, $l = self::length($str); $i < $l; $i++) {
                $chars[] = mb_substr($str, $i, 1, self::$char_set);
            }
        } else {
            $chars = str_split($str);
        }
        
        return $chars;
    }