As we have mentioned previously, PHP, by design, does not do anything except what you tell it to do. It is precisely that hands-off attitude that permits exploits such as the one we just described.
We will assume that you will not knowingly or even accidentally construct a database query that has destructive effects; the problem is with input from your users. Let's therefore look now in more detail at the various ways in which users might provide information to your scripts.
The ways in which users can influence the behavior of your scripts are more, and more complex, than they may appear at first glance.
The most obvious source of user input is of course a text input field in a form. With such a field, ...