从AJAX调用创建PHP对象的实例

I'm playing with using AJAX to perform a simple login function using PHP. I haven't been able to get the AJAX call to successfully create an instance of the Controller class in the login handler file. I feel like I'm just not seeing something that is really basic.

I want to just say thank you in advance for any help! Also, this is not meant to be a real website or a real login script. I understand there are SO many security holes the way it is currently written. I fully intent to add all the bells, whistles, validation, etc. necessary to turn this simple code into something useful as soon as I understand the mistake.

The 4 files that should communicate with each other are as follows:

1. view/login.php

   function signIn( )
   {
       if(checkString( ))
       {
           $.ajax({
               type: "POST",
               url: "ajax.php",
               data: "username=" + $("#username").val( ) + "&password=" + $("#password").val( ),
               dataType: "html",
               async: false,
               cache: false,
               success: function(result){
                   $("#temp_container").html(result);
               }
           });
       }
   }
   

2. AJAX.php

   <?php
   if(isset($_POST['username']) && !empty($_POST['password']))
   {
       $controller = new IndexController( );
       $result = $controller->login($_POST['username'], $_POST['password']);
   
       if($result > 0)
           $user_validation = array('true', 'view/chat_app.php');
       else
           $user_validation = array('false', 'index.php?error_num=1');
   
       echo json_encode($user_validation);
   }
   else if(isset($_POST['username']) && empty($_POST['password']))
   {
       //notify the user they didn't put in a password
   }
   ?>
   

3. Controller.php

   <?php
   include_once("model/indexModel.php");
   
   class IndexController 
   {
       public $model;
   
       public function __construct( ) 
       {
           $model = new IndexModel( );
       }
   
       public function login($username, $password)
       {
           $result = $model->login($username, $password);
           if($result >= 1)
               return true;
           else
               return false;
       }
   }
   ?>
   

4. Model.php

   <?php
   include_once("config/config.php");
   $db = new mysqli($GLOBALS['config']['db_host'], $GLOBALS['config']['db_username'], $GLOBALS['config']['db_password'], $GLOBALS['config']['db_name']);
   
   class IndexModel 
   {
       public function login($username, $password) 
       {
           global $db;
   
           $statement = $db->prepare("SELECT 1 FROM authorized_users 
                                      WHERE username = ?
                                            AND password = ?");
           $statement->bind_param('ss', $username, $password);
           $statement->execute( );
           $statement->close( );
   
           return $statement->affected_rows;
       }
   }
   ?>