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 turn on session.auto_start then the only way to put objects into your sessions is to load its class definition using auto_prepend_file in which you load the class definition else you will have to serialize() your object and unserialize() it afterwards.
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 object).
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.
PHP 5.2.2 introduced an undocumented feature to store session files in "/tmp" even if open_basedir was enabled and "/tmp" is not explicitly added to the allowed paths list. This feature has been removed from PHP as of PHP 5.3.0.