(PHP 5 >= 5.4.0)
SessionHandler a file-based session save handler class and is the default implementation for PHP.
This class can be extended, and its methods overridden. It's interface, SessionHandlerInterface, can be implemented by a custom class as well, completely bypassing this default session handler.
An instance of this class, one extending it, or a custom class implementing SessionHandlerInterface, is passed to session_set_save_handler() in order to replace the currently configured session handler.