我发现我的PHP脚本文件中的一些错误及其与PHP脚本的Facebook数据存储

My Code is showed below:

session_start();
require_once __DIR__ . '\Facebook.php';
$fb = new Facebook\Facebook([
  'app_id' => 'App ID',
  'app_secret' => 'App Secreat',
  'default_graph_version' => 'v2.5'
  ]);

The problem is :

Fatal error: Class 'Facebook\Facebook' not found in C:\xampp\htdocs\Facebook.php on line 4

How can i define Class Facebook?