When session.lazy_write is enabled, which is the default behaviour, session data will NOT be UPDATED if it remains unchanged, in this way the WRITE method of the session handler will not be called at all.
If your session handler storages session data into files, UpdateTimestamp is used to update the "last modified time" of the session file, if your session handler storages session data into a database, UpdateTimestamp is used to update the table field that storages the last modified time of the session registry.
