Controlling Program Execution

It is a rare program that simply executes from the beginning to the end, because in most cases there are some conditions that can change the program execution flow. Thus, these conditions have to be checked and reacted to in one way or another. Let's consider an example of a site's main page. When a user visits the site for the first time, he or she can be shown some additional information or greeted with some funny presentation to get him interested in the site. For succeeding visits by the same user, the presentation is no longer shown. The script logic for these actions will be something like the following:

• If visiting for the first time, show the presentation before showing the main page.
• Otherwise, show the main page right away.

As another example, we have to do numerous checks to ensure that a script is reliable and secure. For example, if a script is intended to send a mail message, it is a good idea to check whether the address is specified correctly before mailing the message. Here, the logic can be the following:

• If the address format is valid, mail the message.
• Otherwise, don't and issue an error message.