The success of the web is deeply rooted in the protocol that drives the web: HTTP. HTTP over TCP is by its very nature stateless, which means that inherently the web is also stateless. While this very aspect is one of the dominating factors for why the web has become such a popular medium, it also causes an interesting problem for developers that want to use the web as an application platform.
The act of interacting with a web application is typically defined by the sum of all requests sent to a web server. Since there can be many consumers being served simultaneously, the application must decide which requests belong to which consumer. These requests are typically known as a "session".
In PHP, the session problem is solved by the session extension which
utilizes some state tracking, typically cookies, and some form of local storage which is
exposed via the $_SESSION superglobal. In Zend Framework, the component
Zend_Session adds value to the PHP session
extension making it easier to use and depend on inside object-oriented applications.