Session handling functions

&reftitle.intro; 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 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. 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. Session handling was added in PHP 4.0. 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.

Sessions and security 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.

&reftitle.required; &no.requirement; 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.

&reference.session.configure; &reference.session.ini;

&reftitle.resources; &no.resource;

&reference.session.constants;

&reftitle.examples; 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 $_SESSION, there is no need to use the session_register, session_unregister, session_is_registered functions. Session variables are accessible like any other variables. Registering a variable with <varname>$_SESSION</varname>. ]]> Unregistering a variable with <varname>$_SESSION</varname> and <link linkend="ini.register-globals"><literal>register_globals</literal></link> disabled. ]]> Unregistering a variable with <link linkend="ini.register-globals"><literal>register_globals</literal></link> enabled, after registering it using <varname>$_SESSION</varname>. ]]> 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. Registering a variable with <link linkend="ini.register-globals"><literal>register_globals</literal></link> 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.

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 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. 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. Counting the number of hits of a single user Hello visitor, you have seen this page times.

To continue, click here ]]> The strip_tags is 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. 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.

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.