public function validateIsbn($check)
    {
        $ean = strval($check);
        $code = substr($ean, 0, -1);
        $key = $this->calculateCheckdigit13($code);
        
        // in key steht die prüfziffer - an den code anhängen
        $code .= $key;
        if ($code == $ean)
        {
            return true;
        }else
        {
            return false;
        }
    }