Advisory file locking - Справочник: PHP5.4

public bool streamWrapper::stream_lock ( mode $operation )

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

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. (not supported on Windows)

Возвращает TRUE в случае успешного завершения или FALSE в случае возникновения ошибки.

Emits E_WARNING if call to this method fails (i.e. not implemented).