My suggestion is that you create a client table, for example, and at the moment of login you save the client's login ID in the session. Later, in the queries, insertions, changes ... finally, in the actions in the database, you must validate the client ID. For example:
In the customer table you have the columns: Id, Name, CNPJ, Status.
In the product table you have the columns: Id, Name, Weight, Value, CustomerID.
When you select products, for example, you will use:
SELECT C.Nome AS Clietne, P.Nome AS Produto, P.Peso, P.Valor
FROM Produtos P
INNER JOIN Clientes C ON (P.IdCliente = C.Id)
WHERE P.IdCliente = $_SESSION['sIdCliente']