public function getData($index, $position, $length)
	{
		// if position is not valid return empty string
		if (!isset($this->_list[$index]) || ($position >= $this->_list[$index]->Size) || ($position < 0)) {
			return '';
		}
		$fh = $this->getStream($this->_list[$index]);
		$data = stream_get_contents($fh, $length, $position);
		fclose($fh);
		return $data;
	}