Question

Every time I access data in $_SESSION, Does it immediately update the session file on the disk, or just once when the process goes down? Or every n bytes of data change (flush)?

This question is not necessarily about the specific file session handler, but every handler. (Does every touch in session immediately invoke an I/O of any kind, beside the storing of a normal variable in memory).

Answer 1

As Matt wrote, it writes at the end of script execution by default. You can read about it here in session_write_close()

Session data is usually stored after your script terminated without the need to call session_write_close(), but as session data is locked to prevent concurrent writes only one script may operate on a session at any time. When using framesets together with sessions you will experience the frames loading one by one due to this locking. You can reduce the time needed to load all the frames by ending the session as soon as all changes to session variables are done.