致命错误：在Codeigniter 2和PHP 5.1中找不到类'DOMDocument'

I use PHP 5.6 in my localhost and this script for generating pdf is working well. But in the server which is using PHP 5.1 I got that error. Here is the script in application/libraries/pdf.php.

<?php
defined('BASEPATH') OR exit('No direct script access allowed');
require_once (dirname(__FILE__)) . '/dompdf/dompdf_config.inc.php';

class Pdfgenerator {

   function __construct() {
      $this->ci =& get_instance();

      $this->dompdf = new DOMPDF();

   }

   function generate($data){
      $html = $this->ci->load->view($data['template'],$data,true);
      $paper_size = isset($data['paper_size']) ? $data['paper_size'] : 'A4';
      $orientation = isset($data['orientation']) ? $data['orientation'] : 'potrait';
      $this->dompdf->set_paper($paper_size,$orientation);
      $this->dompdf->load_html($html);
      $this->dompdf->render();
      $this->dompdf->stream($data['filename'].'.pdf',array('Attachment'=>0));                
   }
}

What should I change to make it works in PHP 5.1?