Úvod

Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site.

If you are familiar with the session management of PHPLIB, you will notice that some concepts are similar to PHP's session support.

A visitor accessing your web site is assigned an unique id, the so-called session id. This is either stored in a cookie on the user side or is propagated in the URL.

The session support allows you to register arbitrary numbers of variables to be preserved across requests. When a visitor accesses your site, PHP will check automatically (if session.auto_start is set to 1) or on your request (explicitly through session_start() or implicitly through session_register()) whether a specific session id has been sent with the request. If this is the case, the prior saved environment is recreated.

All registered variables are serialized after the request finishes. Registered variables which are undefined are marked as being not defined. On subsequent accesses, these are not defined by the session module unless the user defines them later.

Sessions and security

Using sessions, does not mean, you can be absolutely sure, that the session data can only be viewed by that user. This is important to keep in mind, when storing and displaying sensitive information. When storing data into a session, one should always ask themselves, what the damage is, when somebody else views that information, or how your application is affected when this session is actually somebody else.

For instance, if somebody else takes a session, can he then post a message in a forum, as that user and how big of a problem is that? Or perhaps he can view what the original user was thinking of ordering, because he gets access to that user's shopping cart. Obviously for a flowershop, this is less dramatic, than for a pharmacy.

Therefore, when dealing with sensitive information, there should always be additional methods to decide whether it is a valid session. Sessions are not reliable as a secure authentication mechanism.

Sessions rely on the session ID, meaning one can 'steal' a session, by stealing the session ID. This can be made harder, by using a cookie specifically a session cookie, but does not in any way make it impossible and still relies on the user closing all browser windows, to expire the session cookie. Besides that, even session cookies can be sniffed on a network or logged by a proxyserver.

Požadavky

Tyto funkce jsou k dispozici jako součást standardního modulu, který je vždy dostupný.

Instalace

Session support is enabled in PHP by default. If you would not like to build your PHP with session support, you should specify the --disable-session option to configure.

Konfigurace běhu

Chování těchto funkcí je ovlivněno nastavením parametrů v php.ini.

The session management system supports a number of configuration options which you can place in your php.ini file. We will give a short overview.

The track_vars and register_globals configuration settings influence how the session variables get stored and restored.

Typy prostředků

Toto rozšíření nemá definován žádný typ prostředku (resource).

Předdefinované konstanty

Tyto konstanty jsou definovány tímto rozšířením a budou k dispozici pouze tehdy, bylo-li rozšíření zkompilováno společně s PHP nebo dynamicky zavedeno za běhu.

Příklady

If register_globals is disabled, only members of the global associative array $_SESSION can be registered as session variables. The restored session variables will only be available in the array $_SESSION.

Use of $_SESSION (or $HTTP_SESSION_VARS with PHP 4.0.6 or less) is recommended for improved security and code readablity. With $_SESSION, there is no need to use the session_register(), session_unregister(), session_is_registered() functions. Session variables are accessible like any other variables.

Příklad 1. Registering a variable with $_SESSION. <?php session_start(); // Use $HTTP_SESSION_VARS with PHP 4.0.6 or less if (!isset($_SESSION['count'])) { $_SESSION['count'] = 0; } else { $_SESSION['count']++; } ?>

Příklad 2. Unregistering a variable with $_SESSION and register_globals disabled. <?php session_start(); // Use $HTTP_SESSION_VARS with PHP 4.0.6 or less unset($_SESSION['count']); ?>

Příklad 3. Unregistering a variable with register_globals enabled, after registering it using $_SESSION. <?php session_start(); // With PHP 4.3 and later, you can also simply use the prior example. session_unregister('count'); ?>

If register_globals is enabled, then each global variable can be registered as session variable. Upon a restart of a session, these variables will be restored to corresponding global variables. Since PHP must know which global variables are registered as session variables, users need to register variables with session_register() function. You can avoid this by simply setting entries in $_SESSION.

<?php
if (!session_is_registered('count')) {
    session_register("count");
    $count = 0;
}
else {
    $count++;
}
?>

If register_globals is enabled, then the global variables and the $_SESSION entries will automatically reference the same value for session variables which were registered in prior session instances.

Additionally, if you register a new session variable by using session_register(), the entry in the global scope and the $_SESSION entry will not reference the same value until the next session_start() (this applies to PHP 4.2 and before only). I.e. a modification to the global variable will not be reflected by the $_SESSION entry. This is unlikely to matter in practice and has been corrected in PHP 4.3.

Passing the Session ID

There are two methods to propagate a session id:

• Cookies

• URL parameter

The session module supports both methods. Cookies are optimal, but since they are not reliable (clients are not bound to accept them), we cannot rely on them. The second method embeds the session id directly into URLs.

PHP is capable of doing this transparently if compiled with --enable-trans-sid. This option is always enabled in PHP 4.2 and later. If you enable this option, relative URIs will be changed to contain the session id automatically. Alternatively, you can use the constant SID which is defined, if the client did not send the appropriate cookie. SID is either of the form session_name=session_id or is an empty string.

The following example demonstrates how to register a variable, and how to link correctly to another page using SID.

Příklad 5. Counting the number of hits of a single user <?php if (!session_is_registered('count')) { session_register('count'); $count = 1; } else { $count++; } ?> Hello visitor, you have seen this page <?php echo $count; ?> times.<p> To continue, <A HREF="nextpage.php?<?php echo SID?>">click here</A>

The <?php echo SID?> (<?=SID?> can be used if short_open_tag is enabled) is necessary to preserve the session id in the case that the user has disabled cookies. The <?=SID?> is not necessary, if --enable-trans-sid was used to compile PHP.

Custom Session Handlers

To implement database storage, or any other storage method, you will need to use session_set_save_handler() to create a set of user-level storage functions.