Why does Ajax return an error after contacting the server?

Question

css

Authorization through VKontakte, why does ajax return an error when making a request to the server?

Ajax is called from indexa.php file

Log:

XMLHttpRequest cannot load http://oauth.vk.com/authorize?client_id=5993817&redirect_uri=http://localhost/&response_type=code. Response for preflight is invalid (redirect)

And here is the index.php server itself:

<?php
$client_id = '5944817'; // ID приложения
$client_secret = 'CjHNgfgCDqfVJ6PrK3pa'; // Защищённый ключ
$redirect_uri = 'http://localhost/'; // Адрес сайта
$msgtoclient = '';

    $url = 'http://oauth.vk.com/authorize';
 
    $params = array(
        'client_id'     => $client_id,
        'redirect_uri'  => $redirect_uri,
        'response_type' => 'code'
    );
  
 $link = $url . '?' . urldecode(http_build_query($params));
 header('Location:'.$link);
 exit;
if (isset($_GET['code'])) {
    $result = false;
    $params = array(
        'client_id' => $client_id,
        'client_secret' => $client_secret,
        'code' => $_GET['code'],
        'redirect_uri' => $redirect_uri
    );
 
    $token = json_decode(file_get_contents('https://oauth.vk.com/access_token' . '?' . urldecode(http_build_query($params))), true);
 
    if (isset($token['access_token'])) {
        $params = array(
            'uids'         => $token['user_id'],
            'fields'       => 'uid,first_name,last_name,screen_name,sex,bdate,photo_big',
            'access_token' => $token['access_token']
        );
 
        $userInfo = json_decode(file_get_contents('https://api.vk.com/method/users.get' . '?' . urldecode(http_build_query($params))), true);
        if (isset($userInfo['response'][0]['uid'])) {
            $userInfo = $userInfo['response'][0];
            $result = true;
        }
    }
 
    if ($result) {
$msgtoclient = $userInfo;
} else {$msgtoclient = 'sdsdsd';}
}
echo json_encode($msgtoclient) ;
?>

Answer 1

https://github.com/jnicol/particleground

Answer 2

I have a couple of similar screenshots on pinterest:
https://pin.it/o7y72wqossmhsh
you can google it and see in the code which script is made

Answer 3

What exactly is the error? What's in the web server logs?