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
Session handling was added in PHP 4.0.0.
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