src/Debug/FileStreamWrapper.php
public function stream_lock($operation)
    {
        if (!$this->handle) {
            return false;
        }
        self::restorePrev();
        $success = \flock($this->handle, $operation);
        self::register();
        return $success;
    }
public function stream_lock($operation)
    {
        if (!$this->handle) {
            return false;
        }
        self::restorePrev();
        $success = \flock($this->handle, $operation);
        self::register();
        return $success;
    }
Advisory file locking

@param integer $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).

@return boolean

@see http://php.net/manual/en/streamwrapper.stream-lock.php