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.
A visitor accessing your web site is assigned a 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.
If you do turn on session.auto_start then you cannot put objects into your sessions since the class definition has to be loaded before starting the session in order to recreate the objects in your session.
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.
Some types of data can not be serialized thus stored in sessions. It includes resource variables or objects with circular references (i.e. objects which passes a reference to itself to another object).
Note: Session handling was added in PHP 4.0.0.
Note: Please note when working with sessions that a record of a session is not created until a variable has been registered using the session_register() function or by adding a new key to the $_SESSION superglobal array. This holds true regardless of if a session has been started using the session_start() function.
External links: » Session fixation
The session module cannot guarantee that the information you store in a session is only viewed by the user who created the session. You need to take additional measures to actively protect the integrity of the session, depending on the value associated with it.
Assess the importance of the data carried by your sessions and deploy additional protections -- this usually comes at a price, reduced convenience for the user. For example, if you want to protect users from simple social engineering tactics, you need to enable session.use_only_cookies. In that case, cookies must be enabled unconditionally on the user side, or sessions will not work.
There are several ways to leak an existing session id to third parties. A leaked session id enables the third party to access all resources which are associated with a specific id. First, URLs carrying session ids. If you link to an external site, the URL including the session id might be stored in the external site's referrer logs. Second, a more active attacker might listen to your network traffic. If it is not encrypted, session ids will flow in plain text over the network. The solution here is to implement SSL on your server and make it mandatory for users.
No external libraries are needed to build this extension.
Note: Optionally you can use shared memory allocation (mm), developed by Ralf S. Engelschall, for session storage. You have to download » mm and install it. This option is not available for Windows platforms. Note that the session storage module for mm does not guarantee that concurrent accesses to the same session are properly locked. It might be more appropriate to use a shared memory based filesystem (such as tmpfs on Solaris/Linux, or /dev/md on BSD) to store sessions in files, because they are properly locked. Session data is stored in memory thus web server restart deletes it.
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. To use shared memory allocation (mm) for session storage configure PHP --with-mm[=DIR] .
The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.
Note: By default, all data related to a particular session will be stored in a file in the directory specified by the session.save_path INI option. A file for each session (regardless of if any data is associated with that session) will be created. This is due to the fact that a session is opened (a file is created) but no data is even written to that file. Note that this behavior is a side-effect of the limitations of working with the file system and it is possible that a custom session handler (such as one which uses a database) does not keep track of sessions which store no data.
The behaviour of these functions is affected by settings in php.ini.
| Name | Default | Changeable | Changelog |
|---|---|---|---|
| session.save_path | "" | PHP_INI_ALL | |
| session.name | "PHPSESSID" | PHP_INI_ALL | |
| session.save_handler | "files" | PHP_INI_ALL | |
| session.auto_start | "0" | PHP_INI_ALL | |
| session.gc_probability | "1" | PHP_INI_ALL | |
| session.gc_divisor | "100" | PHP_INI_ALL | Available since PHP 4.3.2. |
| session.gc_maxlifetime | "1440" | PHP_INI_ALL | |
| session.serialize_handler | "php" | PHP_INI_ALL | |
| session.cookie_lifetime | "0" | PHP_INI_ALL | |
| session.cookie_path | "/" | PHP_INI_ALL | |
| session.cookie_domain | "" | PHP_INI_ALL | |
| session.cookie_secure | "" | PHP_INI_ALL | Available since PHP 4.0.4. |
| session.cookie_httponly | "" | PHP_INI_ALL | Available since PHP 5.2.0. |
| session.use_cookies | "1" | PHP_INI_ALL | |
| session.use_only_cookies | "1" | PHP_INI_ALL | Available since PHP 4.3.0. |
| session.referer_check | "" | PHP_INI_ALL | |
| session.entropy_file | "" | PHP_INI_ALL | |
| session.entropy_length | "0" | PHP_INI_ALL | |
| session.cache_limiter | "nocache" | PHP_INI_ALL | |
| session.cache_expire | "180" | PHP_INI_ALL | |
| session.use_trans_sid | "0" | PHP_INI_ALL | PHP_INI_ALL in PHP <= 4.2.3. PHP_INI_PERDIR in PHP < 5. Available since PHP 4.0.3. |
| session.bug_compat_42 | "1" | PHP_INI_ALL | Available since PHP 4.3.0. Removed in PHP 6.0.0. |
| session.bug_compat_warn | "1" | PHP_INI_ALL | Available since PHP 4.3.0. Removed in PHP 6.0.0. |
| session.hash_function | "0" | PHP_INI_ALL | Available since PHP 5.0.0. |
| session.hash_bits_per_character | "4" | PHP_INI_ALL | Available since PHP 5.0.0. |
| url_rewriter.tags | "a=href,area=href,frame=src,form=,fieldset=" | PHP_INI_ALL | Available since PHP 4.0.4. |
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.
There is an optional N argument to this directive that determines the number of directory levels your session files will be spread around in. For example, setting to '5;/tmp' may end up creating a session file and location like /tmp/4/b/1/e/3/sess_4b1e384ad74619bd212e236e52a5a174If . In order to use N you must create all of these directories before use. A small shell script exists in ext/session to do this, it's called mod_files.sh. Also note that if N is used and greater than 0 then automatic garbage collection will not be performed, see a copy of php.ini for further information. Also, if you use N, be sure to surround session.save_path in "quotes" because the separator (;) is also used for comments in php.ini.
If you leave this set to a world-readable directory, such as /tmp (the default), other users on the server may be able to hijack sessions by getting the list of files in that directory.
Note: Prior to PHP 4.3.6, Windows users had to change this variable in order to use PHP's session functions. A valid path must be specified, e.g.: c:/temp.
Note: If different scripts have different values of session.gc_maxlifetime but share the same place for storing the session data then the script with the minimum value will be cleaning the data. In this case, use this directive together with session.save_path.
Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other filesystem where atime tracking is not available. Since PHP 4.2.3 it has used mtime (modified date) instead of atime. So, you won't have problems with filesystems where atime tracking is not available.
Note: The expiration timestamp is set relative to the server time, which is not necessarily the same as the time in the client's browser.
Note: For PHP 4.1.2 or less, it is enabled by compiling with --enable-trans-sid. From PHP 4.2.0, trans-sid feature is always compiled. URL based session management has additional security risks compared to cookie based session management. Users may send a URL that contains an active session ID to their friends by email or users may save a URL that contains a session ID to their bookmarks and access your site with the same session ID always, for example.
Since PHP 6.0.0 it is also possible to specify any of the algorithms provided by the hash extension (if it is available), like sha512 or whirlpool. A complete list of supported algorithms can be obtained with the hash_algos() function.
Note: This was introduced in PHP 5.
Note: This was introduced in PHP 5.
Note: If you want HTML/XHTML strict conformity, remove the form entry and use the <fieldset> tags around your form fields.
The track_vars and register_globals configuration settings influence how the session variables get stored and restored.
Note: As of PHP 4.0.3, track_vars is always turned on.
This extension has no resource types defined.
The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
Note: As of PHP 4.1.0, $_SESSION is available as a global variable just like $_POST, $_GET, $_REQUEST and so on. Unlike $HTTP_SESSION_VARS, $_SESSION is always global. Therefore, you do not need to use the global keyword for $_SESSION. Please note that this documentation has been changed to use $_SESSION everywhere. You can substitute $HTTP_SESSION_VARS for $_SESSION, if you prefer the former. Also note that you must start your session using session_start() before use of $_SESSION becomes available.
The keys in the $_SESSION associative array are subject to the same limitations as regular variable names in PHP, i.e. they cannot start with a number and must start with a letter or underscore. For more details see the section on variables in this manual.
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 readability. 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.
Example#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']++;
}
?>
Example#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']);
?>
Do NOT unset the whole $_SESSION with unset($_SESSION) as this will disable the registering of session variables through the $_SESSION superglobal.
You can't use references in session variables as there is no feasible way to restore a reference to another variable.
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.
Before PHP 4.3.0, if you are using $_SESSION and you have disabled register_globals, don't use session_register(), session_is_registered() or session_unregister(). Disabling register_globals is recommended for both security and performance reasons.
If register_globals is enabled, then the global variables and the $_SESSION entries will automatically reference the same values which were registered in the prior session instance. However, if the variable is registered by $_SESSION then the global variable is available since the next request.
There is a defect in PHP 4.2.3 and earlier. 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(). I.e. a modification to the newly registered global variable will not be reflected by the $_SESSION entry. This has been corrected in PHP 4.3.0.
There are two methods to propagate a session id:
The session module supports both methods. Cookies are optimal, but because they are not always available, we also provide an alternative way. The second method embeds the session id directly into URLs.
PHP is capable of transforming links transparently. Unless you are using PHP 4.2.0 or later, you need to enable it manually when building PHP. Under Unix, pass --enable-trans-sid to configure. If this build option and the run-time option session.use_trans_sid are enabled, relative URIs will be changed to contain the session id automatically.
Note: The arg_separator.output php.ini directive allows to customize the argument seperator. For full XHTML conformance, specify & there.
Alternatively, you can use the constant SID which is defined if the session started. If the client did not send an appropriate session cookie, it has the form session_name=session_id. Otherwise, it expands to an empty string. Thus, you can embed it unconditionally into URLs.
The following example demonstrates how to register a variable, and how to link correctly to another page using SID.
Example#3 Counting the number of hits of a single user
<?php
session_start();
if (empty($_SESSION['count'])) {
$_SESSION['count'] = 1;
} else {
$_SESSION['count']++;
}
?>
<p>
Hello visitor, you have seen this page <?php echo $_SESSION['count']; ?> times.
</p>
<p>
To continue, <a href="nextpage.php?<?php echo htmlspecialchars(SID); ?>">click
here</a>.
</p>
The htmlspecialchars() may be used when printing the SID in order to prevent XSS related attacks.
Printing the SID, like shown above, is not necessary if --enable-trans-sid was used to compile PHP.
Note: Non-relative URLs are assumed to point to external sites and hence don't append the SID, as it would be a security risk to leak the SID to a different server.
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.