src/Debug/FileStreamWrapper.php
public function stream_flush()
    {
        if (!$this->handle) {
            return false;
        }
        self::restorePrev();
        $success = \fflush($this->handle);
        self::register();
        return $success;
    }
public function stream_flush()
    {
        if (!$this->handle) {
            return false;
        }
        self::restorePrev();
        $success = \fflush($this->handle);
        self::register();
        return $success;
    }
Flush the output

@return boolean

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