在codeigniter中获取服务器中的dbutil（）错误

I am trying to take db backup in codeigniter using function dbutil. It is working fine in local but in live server i am getting some php error. Please help me where i did wrong.

below is my code and error

Controller

function db_backup()
{
   $this->load->dbutil();

   $prefs = array(     
                    'format'      => 'zip',             
                    'filename'    => 'industry_speak.sql'
                  );

   $backup =& $this->dbutil->backup($prefs); 

   $db_name = 'is-backup-on-'. date("Y-m-d-H-i-s") .'.zip';
   $save = './assets/db_backup/'.$db_name;

   $this->load->helper('file');
   write_file($save, $backup); 

   $this->load->helper('download');

   force_download($db_name, $backup);

}   

Error

In line No. 3775 below code is present.

$backup =& $this->dbutil->backup($prefs); 

A PHP Error was encountered

Severity: Runtime Notice

Message: Only variables should be assigned by reference

Filename: controllers/Admin.php

Line Number: 3775

Backtrace:

File: /home/acetronsma/public_html/application/controllers/Admin.php Line: 3775 Function: _error_handler

File: /home/acetronsma/public_html/index.php Line: 316 Function: require_once

$backup = $this->dbutil->backup($prefs);

remove =&,

cm