Custom Session Handlers

To implement database storage, or any other storage method, you will need to use session_set_save_handler() to create a set of user-level storage functions. As of PHP 5.4.0 you may create session handlers using the SessionHandlerInterface or extend internal PHP handlers by inheriting from SessionHandler.

The callbacks specified in session_set_save_handler() are methods called by PHP during the life-cycle of a session: open, read, write and close and for the housekeeping tasks: destroy for deleting a session and gc for periodic garbage collection.

Therefore, PHP always requires session save handlers. The default is usually the internal 'files' save handler. A custom save handler can be set using session_set_save_handler(). Alternative internal save handlers are also provided by PHP extensions, such as sqlite, memcache and memcached and can be set with session.save_handler.

When the session starts, PHP will internally call the open handler followed by the read callback which should return an encoded string exactly as it was originally passed for storage. Once the read callback returns the encoded string, PHP will decode it and then populate the resulting array into the $_SESSION superglobal.

When PHP shuts down (or when session_write_close() is called), PHP will internally encode the $_SESSION superglobal and pass this along with the session ID to the write callback. After the write callback has finished, PHP will internally invoke the close callback handler.

When a session is specifically destroyed, PHP will call the destroy handler with the session ID.

PHP will call the gc callback from time to time to expire any session records according to the set max lifetime of a session. This routine should delete all records from persistent storage which were last accessed longer than the $lifetime.