【CodeIgniter3】ガラケーの時だけ文字コードをShift_JISに変更する方法

welcome.php

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

class Welcome extends CI_Controller {

	/**
	 * Index Page for this controller.
	 *
	 * Maps to the following URL
	 * 		http://example.com/index.php/welcome
	 *	- or -
	 * 		http://example.com/index.php/welcome/index
	 *	- or -
	 * Since this controller is set as the default controller in
	 * config/routes.php, it's displayed at http://example.com/
	 *
	 * So any other public methods not prefixed with an underscore will
	 * map to /index.php/welcome/<method_name>
	 * @see https://codeigniter.com/user_guide/general/urls.html
	 */
	
	public function index()
	{
		$this->_display('welcome_message');
	}
	
	private function _display($view, $vars = [])
	{
		$output = $this->load->view($view, $vars, TRUE);
	
		if($this->_get_device_type() == 'featurephone'){
			$this->output->set_header('Content-Type: text/html; charset=Shift_JIS');
			$output = mb_convert_encoding($output, "SJIS", "UTF-8");
		}
		
		$this->output->set_output($output);
	}
	
	private function _get_device_type()
	{
		$this->load->library(['user_agent']);
	  
		if(
			strpos($this->agent->agent_string(), 'DoCoMo') !== FALSE ||
			strpos($this->agent->agent_string(), 'KDDI') !== FALSE ||
			strpos($this->agent->agent_string(), 'Vodafone') !== FALSE ||
			strpos($this->agent->agent_string(), 'SoftBank') !== FALSE ||
			strpos($this->agent->agent_string(), 'DDIPOKET') !== FALSE ||
			strpos($this->agent->agent_string(), 'UP.Browser') !== FALSE ||
			strpos($this->agent->agent_string(), 'WILLCOM') !== FALSE ||
			strpos($this->agent->agent_string(), 'J-PHONE') !== FALSE ||
			strpos($this->agent->agent_string(), 'emobile') !== FALSE
		)
		{
			return "featurephone";
		}
		else if ($this->agent->is_mobile())
		{
			return "smartphone";
		}
		else
		{
			return "pc";
		}
	}
}