'Class 'Facebook\Facebook' not found" Facebook SDK error

I think you need to first import that php class into your current file

Put this line at the top

use Facebook\Facebook; 

Download the zip file from here : https://github.com/facebook/php-graph-sdk/archive/5.4.zip

Steps :

1. Unzip the content (by either just double clicking on the zipped file or use any available unzipping software to unzip or decompress the downloaded file)

2. Navigate to the "src" folder.

3. Copy or cut the "src" folder and paste right within the folder from which you have your php files.

4. make sure you have created "includes.php" file within your php project workspace ie. the folder in which your web page is sitting. And add this line to your "includes.php" file :

   require_once 'src/Facebook/autoload.php';

5. Now in your php file(s) that has to do with facebook you can then add :

   require_once("includes.php");

6. Now save your file and go into your browser and refresh.

//Do remember to keep your work organised by now referencing all files that may be needed in your project via "includes.php". This may vary for some developers, depending on how and what you are working on.

You need to include the autoloader first to get access to the service methods and classes (as said in the PHP SDK Documentation for Facebook API. You are trying to use a namespaced class Facebook\Facebook, to use its methods, but you don't have the class in the PHP file.

require_once 'src/Facebook/autoload.php';
//Create the Facebook service
$fb = new Facebook\Facebook ([
    'app_id' => '-----------------',
    'app_secret' => '--------------------',
    'default_graph_version' => 'v2.4'
    ]);

Somewhere in your directory (if you installed the Facebook PHP SDK) correctly, you will find the autoload.php file which automatically requires .php files that you need to use the services and methods.