CodeIgniter Convert Array to Json

the php code below uses codeigniter for a select on a db, so the array must then be converted to do so I use the code below but it does not convert the json (the json is reported in gist) therefore it is not cast to a json array.

Gist link of json: https://gist.github.com/riccardopirani/2a48e8e62fa65512fd487de0ca82bc79

Controller.php

class Api2 extends CI_Controller {
    
    function __construct() {
        parent::__construct();
        $this->load->model('Book_model');
    }


    function index() {
        header('Content-Type: application/json');
        //print link element
        $array = array(
            "foo" => "bar",
            "bar" => "foo",
        );
        $values["data"] = $this->Book_model->get_all_books();
        /*foreach($_GET as $key => $value){
            echo $key . " : " . $value . "\n";
        } */
        //Return data
        // $data = json_encode($tempdata);
        print_r(json_encode($values["data"]));
        return (json_encode($values["data"]));

Model.php

<?php

class Book_model extends CI_Model
{
    function __construct()
    {
        parent::__construct();
    }
    
    /*
     * Get book by id
     */
    function get_book($id)
    {
        return $this->db->get_where('books',array('id'=>$id))->row_array();
    }
        
    /*
     * Get all books
     */
    public function get_all_books()
    {
        $this->db->order_by('id', 'desc');
        return $this->db->get('books')->result_array();
    }
        
    /*
     * function to add new book
     */
    function add_book($params)
    {
        $this->db->insert('books',$params);
        return $this->db->insert_id();
    }
    
    /*
     * function to update book
     */
    function update_book($id,$params)
    {
        $this->db->where('id',$id);
        return $this->db->update('books',$params);
    }
    
    /*
     * function to delete book
     */
    function delete_book($id)
    {
        return $this->db->delete('books',array('id'=>$id));
    }
}

source https://stackoverflow.com/questions/67888061/codeigniter-convert-array-to-json