This method is called in response to flock(), when file_put_contents()
(when flags contains LOCK_EX), stream_set_blocking() and when closing the
stream (LOCK_UN).
$operation is one of the following:
LOCK_SH to acquire a shared lock (reader).
LOCK_EX to acquire an exclusive lock (writer).
LOCK_UN to release a lock (shared or exclusive).
LOCK_NB if you don't want flock() to block while locking.