|So only one customer ever can use the site at the same time? You don't have faith in your friend's business, do you?|
Look, it's obvious you're not reading the articles I'm linking, so I'll give you a thought experiment. I'm not drinking coffee, so it's not explained well. That's fine, because the Wikipedia entry explains it better.
1) Customer A reserves a room and hits 'Submit.' codCliente == 5.
2) Customer B reserves a different room and hits 'Submit.' codCliente == 5.
3) You insert the data into slot 5 and increment codCliente for Customer A. codCliente == 6.
4) You insert the data into slot 5 and increment codCliente for Customer B. codCliente == 6.
Now what? If you're lucky enough (read: smart enough to use UNIQUE constraints), you'll error out on Customer B, he'll be frustrated, but tries again. If you're not, you take Customer A's money, but don't reserve his room. If you're really unlucky, you've horribly corrupted your database records, and you'll need to spend all day manually adding and modifying records to get everything where it should be.
How To Ask Questions The Smart Way