A session is a logical object created by the PHP engine to allow you to preserve data across subsequent HTTP requests.There is only one session object available to your PHP scripts at any time.
Data saved to the session by a script can be retrieved by the same script or another script when requested from the same visitor.Sessions are commonly used to store temporary data to allow multiple PHP pages to offer a complete functional transaction for the same visitor.

It can be used to store information on the server for future use.