Facebook SDK致命错误:未捕获的异常'Facebook \ Exceptions \ FacebookSDKException',消息为'Require

At first my code was like this without use namespace but as per the documentation https://developers.facebook.com/docs/php/gettingstarted

    session_start();
    define('FACEBOOK_SDK_V4_SRC_DIR', __DIR__ . '/Facebook/');
    require_once __DIR__ . '/Facebook/autoload.php';



    $fb= new Facebook/Facebook([
    'app_id'                => '******',
    'app_secret'            => '********************',
    'default_graph_version' =>'v2.5'
    ]);

    $fb->getRedirectLoginHelper();
    //$permissions = ['email','id', 'first_name', 'last_name'];
    $loginUrl = $fb->getLoginUrl('index.php');
    echo '<a href="' . $loginUrl . '">Log in with Facebook!</a>';

but it throwed the following error

Fatal error: Class 'Facebook' not found in E:\wamp\www\sugumar\facebooklogin\index.php on line 9 Call Stack

So searched online and found the following link 'Class 'Facebook\Facebook' not found" Facebook SDK error

Which told me include the sdk file and use namespace, since I already included the sdk files, I put use Facebook\Facebook; at the top

so my code becomes

    use Facebook\Facebook; 
    session_start();
    define('FACEBOOK_SDK_V4_SRC_DIR', __DIR__ . '/Facebook/');
    require_once __DIR__ . '/Facebook/autoload.php';



    $fb= new Facebook/Facebook([
    'app_id'                => '********',
    'app_secret'            => '********************',
    'default_graph_version' =>'v2.5'
    ]);

    $fb->getRedirectLoginHelper();
    //$permissions = ['email','id', 'first_name', 'last_name'];
    $loginUrl = $fb->getLoginUrl('index.php');
    echo '<a href="' . $loginUrl . '">Log in with Facebook!</a>';

But now it throws a different error, my appid and secret are correct... I am just trying to change from version 4 to version 5.

The error is

( ! ) Fatal error: Uncaught exception 'Facebook\Exceptions\FacebookSDKException' with message 'Required "app_id" key not supplied in config and could not find fallback environment variable "FACEBOOK_APP_ID"' in E:\wamp\www\sugumar\facebooklogin\Facebook\Facebook.php on line 133

( ! ) Facebook\Exceptions\FacebookSDKException: Required "app_id" key not supplied in config and could not find fallback environment variable "FACEBOOK_APP_ID" in E:\wamp\www\sugumar\facebooklogin\Facebook\Facebook.php on line 133