Introduction

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.

Sessions and security

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.

Installation

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.

Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

Table 1. Session configuration options 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.use_cookies	"1"	PHP_INI_ALL
session.use_only_cookies	"0"	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.
session.bug_compat_warn	"1"	PHP_INI_ALL	Available since PHP 4.3.0.
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.

session.save_handler stringsession.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 stringsession.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. See also session_save_path().

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.

Warning

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.

session.name stringsession.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 booleansession.auto_start specifies whether the session module starts a session automatically on request startup. Defaults to 0 (disabled). session.serialize_handler stringsession.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 integersession.gc_probability in conjunction with session.gc_divisor is used to manage probability that the gc (garbage collection) routine is started. Defaults to 1. See session.gc_divisor for details. session.gc_divisor integersession.gc_divisor coupled with session.gc_probability defines the probability that the gc (garbage collection) process is started on every session initialization. The probability is calculated by using gc_probability/gc_divisor, e.g. 1/100 means there is a 1% chance that the GC process starts on each request. session.gc_divisor defaults to 100. session.gc_maxlifetime integersession.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. 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.

session.referer_check stringsession.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 stringsession.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 integersession.entropy_length specifies the number of bytes which will be read from the file specified above. Defaults to 0 (disabled). session.use_cookies booleansession.use_cookies specifies whether the module will use cookies to store the session id on the client side. Defaults to 1 (enabled). session.use_only_cookies booleansession.use_only_cookies specifies whether the module will only use cookies to store the session id on the client side. Defaults to 0 (disabled, for backward compatibility). Enabling this setting prevents attacks involved passing session ids in URLs. This setting was added in PHP 4.3.0. session.cookie_lifetime integersession.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_path stringsession.cookie_path specifies path to set in session_cookie. Defaults to /. See also session_get_cookie_params() and session_set_cookie_params(). session.cookie_domain stringsession.cookie_domain specifies the domain to set in session_cookie. Default is none at all. See also session_get_cookie_params() and session_set_cookie_params(). session.cookie_secure booleansession.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 stringsession.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 integersession.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 booleansession.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 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.

session.bug_compat_42 booleanPHP versions 4.2.3 and lower have an undocumented feature/bug that allows you to initialize a session variable in the global scope, albeit register_globals is disabled. PHP 4.3.0 and later will warn you, if this feature is used, and if session.bug_compat_warn is also enabled. This feature/bug can be disabled by disabling this directive. session.bug_compat_warn booleanPHP versions 4.2.3 and lower have an undocumented feature/bug that allows you to initialize a session variable in the global scope, albeit register_globals is disabled. PHP 4.3.0 and later will warn you, if this feature is used by enabling both session.bug_compat_42 and session.bug_compat_warn. session.hash_function integersession.hash_function allows you to specify the hash algorithm used to generate the session IDs. \\\\'0\\\\' means MD5 (128 bits) and \\\\'1\\\\' means SHA-1 (160 bits).

Note: This was introduced in PHP 5.

session.hash_bits_per_character integersession.hash_bits_per_character allows you to define how many bits are stored in each character when converting the binary hash data to something readable. The possible values are \\\\'4\\\\' (0-9, a-f), \\\\'5\\\\' (0-9, a-v), and \\\\'6\\\\' (0-9, a-z, A-Z, "-", ",").

Note: This was introduced in PHP 5.

url_rewriter.tags stringurl_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,fieldset=

Note: If you want XHTML conformity, remove the form entry and use the
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.

Predefined Constants

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.

SID (string)Constant containing either the session name and session ID in the form of "name=ID" or empty string if session ID was set in an appropriate session cookie.

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.

Note: The arg_separator.output php.ini directive allows to customize the argument seperator. For full XHTML conformance, specify & there.