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8.6. Using Session Tracking

8.6.3. Discussion

To start a session automatically on each request, set session.auto_start to 1 in php.ini. With session.auto_start, there's no need to call session_start( ).

The session functions keep track of users by issuing them cookies with a randomly generated session IDs. If PHP detects that a user doesn't accept the session ID cookie, it automatically adds the session ID to URLs and forms.[6] For example, consider this code that prints a URL:

[6]Before PHP 4.2.0, this behavior had to be explicitly enabled by building PHP with the --enable-trans-sid configuration setting.

print '<a href="train.php">Take the A Train</a>';

If sessions are enabled, but a user doesn't accept cookies, what's sent to the browser is something like:

<a href="train.php?PHPSESSID=2eb89f3344520d11969a79aea6bd2fdd">Take the A Train</a>

In this example, the session name is PHPSESSID and the session ID is 2eb89f3344520d11969a79aea6bd2fdd. PHP adds those to the URL so they are passed along to the next page. Forms are modified to include a hidden element that passes the session ID. Redirects with the Location header aren't automatically modified, so you have to add a session ID to them yourself using the SID constant:

$redirect_url = 'http://www.example.com/airplane.php';
if (defined('SID') && (! isset($_COOKIE[session_name()]))) {
    $redirect_url .= '?' . SID;

header("Location: $redirect_url");

The session_name( ) function returns the name of the cookie that the session ID is stored in, so this code appends the SID constant only to $redirect_url if the constant is defined, and the session cookie isn't set.

By default, PHP stores session data in files in the /tmp directory on your server. Each session is stored in its own file. To change the directory in which the files are saved, set the session.save_path configuration directive in php.ini to the new directory. You can also call session_save_path( ) with the new directory to change directories, but you need to do this before accessing any session variables.

8.6.4. See Also

Documentation on session_start( ) at http://www.php.net/session-start, session_save_path( ) at http://www.php.net/session-save-path; the session module has a number of configuration directives that help you do things like manage how long sessions can last and how they are cached; these are detailed in the "Sessions" section of the online manual at http://www.php.net/session.

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