private function _closePipe($pipeNumber)
    {
        $pipeNumber = intval($pipeNumber);
        if (array_key_exists($pipeNumber, $this->_openPipes)) {
            fflush($this->_openPipes[$pipeNumber]);
            fclose($this->_openPipes[$pipeNumber]);
            unset($this->_openPipes[$pipeNumber]);
        }
    }