$hook['post_controller_constructor'][] = array(
                                'class'    => 'Mobile',
                                'function' => 'view_set',
                                'filename' => 'Mobile.php',
                                'filepath' => 'hooks'
                                );

$hook['display_override'][] = array(
                                'class'    => 'Mobile',
                                'function' => 'display',
                                'filename' => 'Mobile.php',
                                'filepath' => 'hooks'
                                );

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Mobile {

 function Mobile()
 {
    $this->ci =& get_instance();
    $this->ci->load->library('user_agent');
 }

  function view_set()
  {
    if ($this->ci->agent->is_mobile())
    {
      $this->ci->load->_ci_view_path .= 'mobile/';
    }
  }

  function display()
  {
    if ($this->ci->agent->is_mobile())
    {
      $buffer = $this->ci->output->get_output();

      $buffer = preg_replace("/\n/", '', $buffer);
      $buffer = mb_convert_kana($buffer, 'aks', "UTF-8");
      $buffer = mb_convert_encoding($buffer, 'SJIS', $this->ci->config->item('charset'));

      header("Content-type: text/html; charset=Shift_JIS;");
      echo $buffer;
    }
    else
    {
      echo $this->ci->output->get_output();
    }
  }

}