CodeIgniter：我在这里遇到的唯一错误是我看不到json的响应

Question

控制器

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

class ajax_process extends CI_Controller {

public function __construct() {
    parent::__construct();
    ini_set('session.cookie_httponly', 1); 
    ini_set('session.use_only_cookies', 1);
    $this->load->model('users_class');
}
public function index()
{
    if (!$this->input->is_ajax_request()) {
        exit('No direct script access allowed');
    }
    try {

        $post = $this->input->post();

        switch($post['act']) {

            case 'register':
                $callback = $this->users_class->insert($post);
            break;

            default: 
                $callback = array('status'=>'Invalid action');
        }
        echo json_encode($callback);
    }
    catch(Exception $e) {
        echo json_encode(array('status'=>$e));
    }   
}

}

模型

    if( $chk_strUserName === TRUE )
    {
        $msg['status'] = 'username_exist';
    }
    else if( $chk_strEmail === TRUE )
    {
        $msg['status'] = 'email_exist';
    }
    else if( !valid_email($strEmail) )
    {
        $msg['status'] = 'email_not_valid';
    }
    else
    {
        $query = $this->db->insert('users', $data);
        if($this->db->_error_number()==0){
            $msg['status'] = 'success';
        }
        else
        {
            $msg['status'] = 'error';
        }
    }
    return $msg;

AJAX

    var strFirstName = $('#strFirstName').val();
    var strLastName = $('#strLastName').val();
    var strEmail = $('#strEmail').val();
    var strPassword = $('#strPassword').val();
    var strGender = $('#strGender').val();

    uri = 'ajax_process';
    data = {
        'act' : 'register',
        'strFirstName' : strFirstName,
        'strLastName' : strLastName,
        'strEmail' : strEmail,
        'strPassword' : strPassword,
        'strGender' : strGender
    };
    $.ajax({
        type: "POST",
        url: base_url+uri,
        data: data,
        dataType: 'json',
        success: function(result) {    
            console.log(result);
        }
    });

任何人都可以帮我找到程序中可能存在的错误/错误。

它正在工作，它将数据存储在我的数据库中，但我无法从json接收任何回调