Code igniter API Header setting for JSON output

Api.php

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

class Api extends CI_Controller {


function __construct(){
	parent:: __construct();
$this->output->set_header("Access-Control-Allow-Origin: * ");
$this->output->set_header("Access-Control-Expose-Headers: Access-Control-Allow-Origin");
$this->output->set_header("HTTP/1.0 200 OK");
$this->output->set_header("HTTP/1.1 200 OK");
$this->output->set_header('Last-Modified: '.gmdate('D, d M Y H:i:s', time()).' GMT');
$this->output->set_header("Cache-Control: no-store, no-cache, must-revalidate");
$this->output->set_header("Cache-Control: post-check=0, pre-check=0");
$this->output->set_header("Pragma: no-cache");
$this->output->set_content_type('application/json');
}

  public function index(){
    $this->output->set_output(json_encode($data));
  }
}