Jak změnit číslo objednávky na číselný kód – variabilní symbol (Order Reference)

public static function generateReference()
{
    return strtoupper(Tools::passwdGen(9, 'NO_NUMERIC'));
}

public static function generateReference()
{
    $last_id = Db::getInstance()->getValue('
        SELECT MAX(id_order)
        FROM '._DB_PREFIX_.'orders');
    return str_pad((int)$last_id + 1, 9, '000000000', STR_PAD_LEFT);
 
}

public static function generateReference()
    {
        //return strtoupper(Tools::passwdGen(9, 'NO_NUMERIC'));
        $last_id = Db::getInstance()->getValue('
        SELECT MAX(id_order)
        FROM '._DB_PREFIX_.'orders');
        return (int)$last_id + 1;        
    }