$ vi controllers/Api.php 
<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Api extends CI_Controller {

        public function index()
        {
                $resp = [];
                $resp['a'] = 1;
                $this->output->set_content_type('application/json')->set_output(json_encode($resp));
        }
}

ブラウザでアクセスしてみる

{"a":1}