session_set_save_handler() sets the user-level session storage functions which are used for storing and retrieving data associated with a session. This is most useful when a storage method other than those supplied by PHP sessions is preferred. i.e. Storing the session data in a local database.这个函数是用于设置与session操作相关的处理函数的。
php的session是在服务器端的。
session默认是基于cookie的
session id通过http请求头的cookie项发送过来
你也可以通过url,http实体(post数据)等把session id发送至服务器。