(PHP 5 >= 5.4.0, PHP 7, PHP 8)
SessionHandlerInterface::open — Initialize session
Re-initialize existing session, or creates a new one. Called when a session starts or when session_start() is invoked.
pathThe path where to store/retrieve the session.
nameThe session name.
The return value (usually true on success, false on failure). Note this value is returned internally to PHP for processing.
The suggestion that you should free the lock as soon as possible is WRONG (and for some reason, I can't downvote it right now).
Releasing the lock before the write() call is as effective as not using locks at all. The whole point is that a concurrent read() HAS to be blocked until the session is closed, otherwise you'll have race conditions.
If you care about the performance aspect, you should take care to call session_write_close() as soon as possible instead.Note that once $sessionName has been used to provide a value for $sessionId from the cookie data it is totally redundant as all further reading and writing of the session data is controlled by $sessionId.
If, for any reason, it becomes necessary to identify the value for $sessionName which is associated with the current $sessionId then you should use the value that was passed on the open() method. Attempting to use a value from an alternative source could have unexpected side effects.