private function generateAccountNumber()
    {
        $query = "SELECT account_number FROM accounts ORDER BY account_number DESC LIMIT 1" ;

        $result = $this->mysqli->query($query);


        $num = $result->num_rows;

        if ($num == 0)
            return '12345678900000000001';

        $row = $result->fetch_object();


        $acc = (string)$row->account_number;

        $idx = 11;

        while ($acc[$idx] == '0')
            $idx++;

        $number = substr($acc, $idx, strlen($acc) - $idx);

        $number++;

        $number_len = strlen($number);

        return  substr($acc, 0, strlen($acc) - $number_len) . $number;
    }