Xử lý cộng trừ tiền trong tài khoản khi code bằng CodeIgniter

function withdraw($user_id, $amount) {
    $amount = (int)$amount;

    $this->db->trans_start();

    $query = $this->db->query("SELECT * FROM bank_account WHERE user_id = $user_id AND balance >= $amount FOR UPDATE");

    if($query->num_rows() === 0) {
        $this->db->trans_complete();
        return false;
    }

    $this->db->where('user_id', $user_id)
             ->set('balance', 'balance-'.$amount, false)
             ->update('bank_account');

    $this->db->where('user_id', $user_id)
             ->set('balance', 'balance+'.$amount, false)
             ->update('wallet');

    $this->db->trans_complete();

    return true;
}