public function base64EncodeWrapMB($str, $linebreak = null)
    {
        $start = '=?' . $this->CharSet . '?B?';
        $end = '?=';
        $encoded = '';
        if ($linebreak === null) {
            $linebreak = $this->LE;
        }

        $mb_length = mb_strlen($str, $this->CharSet);
        // Each line must have length <= 75, including $start and $end
        $length = 75 - strlen($start) - strlen($end);
        // Average multi-byte ratio
        $ratio = $mb_length / strlen($str);
        // Base64 has a 4:3 ratio
        $avgLength = floor($length * $ratio * .75);

        for ($i = 0; $i < $mb_length; $i += $offset) {
            $lookBack = 0;
            do {
                $offset = $avgLength - $lookBack;
                $chunk = mb_substr($str, $i, $offset, $this->CharSet);
                $chunk = base64_encode($chunk);
                $lookBack++;
            } while (strlen($chunk) > $length);
            $encoded .= $chunk . $linebreak;
        }

        // Chomp the last linefeed
        $encoded = substr($encoded, 0, -strlen($linebreak));
        return $encoded;
    }