Beginning PHP 5.3

Chapter 10. Preserving State With Query Strings, Cookies, and Sessions

Most of the PHP scripts you created in previous chapters are very much one-shot affairs. Each time they run, they start with a "clean slate" of variables and other data. This is because each request that a browser makes to a Web server is independent of any previous requests. When a Web server receives a request to run a PHP script, it loads the script into its memory, runs it, then removes all trace of it from memory.

However, most of the Web applications you use today have a need to store data between browser requests. For example, a shopping cart needs to remember which items you have added to your cart, and a forum application needs to remember your identity whenever you post a message in the forum.

In other words, there is a need to preserve the current state of a user's interaction with an application from one request to the next.

You've already looked at a simple example of storing state in the previous chapter, when you used hidden form fields to store previously entered form data across each step of a three-stage registration form. Although filling in the registration form involved three separate browser requests — and therefore three separate runs of the PHP script — the script was able to "remember" the state of the registration process by storing it in the forms themselves.

Although this approach works perfectly well for simple cases, it has a few disadvantages. For example, it's a slow way to store large ...