Web Database Applications with PHP and MySQL, 2nd Edition

PHP Session Management

Developing applications that use PHP sessions is straightforward. The three important features of session management—identifying sessions, storing session variables, and cleaning up old sessions—are mostly taken care of by the PHP session management library.

In this section, we show you how sessions are started and ended, and how session variables are used, and provide strategies for designing session-based applications.

The out-of-the-box configuration of PHP session management uses disk-based files to store session variables, and our discussion in this section assumes this default PHP 4.3 behavior. Using files as the session store is adequate for most applications in which the number of concurrent sessions is limited. A more scalable solution that uses a MySQL database as a session store is provided in Appendix F.

Starting a Session

The session_start( ) function is used to create a new session. A session is unique to the interaction between a browser and a web database application. If you use your browser to access several sites at once, you'll have several unrelated sessions. Similarly, if several users access your application each has their own session. However, if you access an application using two browsers (or two browser windows) at the same time, in most cases the browsers will share the same session; this can lead to unpredictable behavior—that's the reason why many web sites warn against it.

The first time a user requests a script that calls session_start( ...