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.
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.
Note: As of PHP 4.1.0, $_SESSION is available as global variable just like $_POST, $_GET, $_REQUEST and so on. Not like $HTTP_SESSION_VARS, $_SESSION is always global. Therefore, global should not be used for $_SESSION.
If track_vars is enabled and register_globals is disabled, only members of the global associative array $HTTP_SESSION_VARS can be registered as session variables. The restored session variables will only be available in the array $HTTP_SESSION_VARS.
Example 1. Registering a variable with track_vars enabled
Use of $_SESSION (or $HTTP_SESSION_VARS with PHP 4.0.6 or less) is recommended for security and code readablity. With $_SESSION or $HTTP_SESSION_VARS, there is no need to use session_register()/session_unregister()/session_is_registered() functions. Users can access session variable like a normal variable.
If register_globals is enabled, then all global variables can be registered as session variables and the session variables will be restored to corresponding global variables. Since PHP must know which global variables are registered as session variables, users must register variables with session_register() function while $HTTP_SESSION_VARS/$_SESSION does not need to use session_register().
If you are using $HTTP_SESSION_VARS/$_SESSION and disable register_globals, do not use session_register(), session_is_registered() and session_unregister().
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 reason.
Example 4. Registering a variable with register_globals enabled
If both track_vars and register_globals are enabled, then the globals variables and the $HTTP_SESSION_VARS/$_SESSION entries will reference the same value for already registered variables.
If user use session_register() to register session variable, $HTTP_SESSION_VARS/$_SESSION will not have these variable in array until it is loaded from session storage. (i.e. until next request)
There are two methods to propagate a session id:
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 when compiled with --enable-trans-sid. 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.
Example 5. Counting the number of hits of a single user
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.
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.
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.
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.
session.name specifies the name of the session which is used as cookie name. It should only contain alphanumeric characters. Defaults to PHPSESSID.
session.auto_start specifies whether the session module starts a session automatically on request startup. 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.
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.
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_path specifies path to set in session_cookie. Defaults to /.
session.cookie_domain specifies domain to set in session_cookie. Default is none at all.
session.cache_limiter specifies cache control method to use for session pages (none/nocache/private/private_no_expire/public). Defaults to nocache.
session.cache_expire specifies time-to-live for cached session pages in minutes, this has no effect for nocache limiter. Defaults to 180.
session.use_trans_sid whether transparent sid support is enabled or not if enabled by compiling with --enable-trans-sid. Defaults to 1 (enabled).
url_rewriter.tags spefifies 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: Session handling was added in PHP 4.0.