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.
Note: Session handling was added in PHP 4.0.
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 addditional 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 appropiate 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 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.
The behaviour of these functions is affected by settings in php.ini.
Table 1. Session configuration options
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.
session.save_handler defines the name of the handler which is used for storing and retrieving data associated with a session. Defaults to files. See also session_set_save_handler().
session.save_path defines the argument which is passed to the save handler. If you choose the default files handler, this is the path where the files are created. Defaults to /tmp. If session.save_path's path depth is more than 2, garbage collection will not be performed. See also session_save_path().
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: Windows users have to change this variable in order to use PHP's session functions. Make sure to specify a valid path, e.g.: c:/temp.
session.name specifies the name of the session which is used as cookie name. It should only contain alphanumeric characters. Defaults to PHPSESSID. See also session_name().
session.auto_start specifies whether the session module starts a session automatically on request startup. Defaults to 0 (disabled).
session.serialize_handler defines the name of the handler which is used to serialize/deserialize data. Currently, a PHP internal format (name php) and WDDX is supported (name wddx). WDDX is only available, if PHP is compiled with WDDX support. Defaults to php.
session.gc_probability specifies the probability that the gc (garbage collection) routine is started on each request in percent. Defaults to 1.
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up.
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 fs where atime tracking is not available.
session.referer_check contains the substring you want to check each HTTP Referer for. If the Referer was sent by the client and the substring was not found, the embedded session id will be marked as invalid. Defaults to the empty string.
session.entropy_file gives a path to an external resource (file) which will be used as an additional entropy source in the session id creation process. Examples are /dev/random or /dev/urandom which are available on many Unix systems.
session.entropy_length specifies the number of bytes which will be read from the file specified above. Defaults to 0 (disabled).
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. The value 0 means "until the browser is closed." Defaults to 0.See also session_get_cookie_params() and session_set_cookie_params().
session.cookie_secure specifies whether cookies should only be sent over secure connections. Defaults to off. This setting was added in PHP 4.0.4. See also session_get_cookie_params() and session_set_cookie_params().
session.cache_limiter specifies cache control method to use for session pages (none/nocache/private/private_no_expire/public). Defaults to nocache. See also session_cache_limiter().
session.cache_expire specifies time-to-live for cached session pages in minutes, this has no effect for nocache limiter. Defaults to 180. See also session_cache_expire().
session.use_trans_sid whether transparent sid support is enabled or not. Defaults to 0 (disabled).
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 an URL that contains an active session ID to their friends by email or users may save an URL that contains a session ID to their bookmarks and access your site with the same session ID always, for example.
url_rewriter.tags specifies which html tags are rewritten to include session id if transparent sid support is enabled. Defaults to a=href,area=href,frame=src,input=src,form=fakeentry
Note: As of PHP 4.0.3, track_vars is always turned on.
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.
Constant containing the session name and session ID in the form of "name=ID".
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 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.
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.
If you are using $_SESSION and disable register_globals, do not use session_register(), session_is_registered() and session_unregister(), if your scripts shall work in PHP 4.2 and earlier. You can use these functions in 4.3 and later.
If you enable register_globals, session_unregister() should be used since session variables are registered as global variables when session data is deserialized. Disabling register_globals is recommended for both security and performance reasons.
Example 4. Registering a variable with register_globals enabled
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.
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.
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 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 always defined. 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 5. Counting the number of hits of a single user
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.
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.