(PHP 5 >= 5.4.0, PHP 7)
SessionHandlerInterface::open — Initialize session
$save_path
, string $session_name
) : boolRe-initialize existing session, or creates a new one. Called when a session starts or when session_start() is invoked.
save_pathThe path where to store/retrieve the session.
session_nameThe session name.
会话存储的返回值（通常成功返回 0，失败返回 1）。
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.
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.