Error: Class 'Facebook\FacebookSession' not found with the facebook PHP SDK

Question 1

I found the solution here

I did not code in php for some time now and things have changed. use Facebook\FacebookSession; is not enough. You need to add a require_once too.

require_once( 'Facebook/FacebookSession.php' );

Edit: for a more detailed solution, please checkout the answer below.

Question 2

PHP Version 5.4.0 or higher is required.
Facebook uses Implementations of PSR-4. Hence You dont have to use require or require_once or include or include_once.
In PSR-4, you just need packagename(namespace) i.e. directory name and class file name only.It will register classes dynamically from given package name.Ex.:- use packaname\classname.
You will find the file autoload.php in Facebook SDK Autoload root directory.
use is used to load dynamic classes using spl_autoload_register
Facebook register all library using autoload.php or autoload_fb.php
You have to find autoload.php in your downloaded library like facebook-php-sdk-v4-4.0-dev/.
If you just wants to use Facebook library from download source.Then you have to copy autoload.php in your root directory or in Facebook directory.
defined constant for FACEBOOK_SDK_V4_SRC_DIR i.e. path of the facebook library
You need to do as below to use in php

Note: I have copied /var/www/stack/24006673/facebook-php-sdk-v4-4.0-dev/src/Facebook directory and /var/www/stack/24006673/facebook-php-sdk-v4-4.0-dev/autoload.php file in root directory /var/www/stack/24006673/

define('FACEBOOK_SDK_V4_SRC_DIR','/var/www/stack/24006673/Facebook/');
require_once("autoload.php");
use Facebook\FacebookSession;
use Facebook\FacebookRequest;
use Facebook\GraphUser;
use Facebook\FacebookRequestException;
use Facebook\FacebookRedirectLoginHelper;
FacebookSession::setDefaultApplication('YOUR_APP_ID','YOUR_APP_SECRET');

Question 3

This code worked for me

session_start();

require_once( 'Facebook/FacebookSession.php' );
require_once( 'Facebook/FacebookRedirectLoginHelper.php' );
require_once( 'Facebook/FacebookRequest.php' );
require_once( 'Facebook/FacebookResponse.php' );
require_once( 'Facebook/FacebookSDKException.php' );
require_once( 'Facebook/FacebookRequestException.php' );
require_once( 'Facebook/FacebookAuthorizationException.php' );
require_once( 'Facebook/GraphObject.php' );
require_once( 'Facebook/Entities/AccessToken.php' );
require_once( 'Facebook/HttpClients/FacebookHttpable.php' );
require_once( 'Facebook/HttpClients/FacebookCurlHttpClient.php' );
require_once( 'Facebook/HttpClients/FacebookCurl.php' ); 

use Facebook\FacebookSession;
use Facebook\FacebookRedirectLoginHelper;
use Facebook\FacebookRequest;
use Facebook\FacebookResponse;
use Facebook\FacebookSDKException;
use Facebook\FacebookRequestException;
use Facebook\FacebookAuthorizationException;
use Facebook\GraphObject;
use Facebook\Entities\AccessToken;
use Facebook\HttpClients\FacebookHttpable;
use Facebook\HttpClients\FacebookCurlHttpClient;
use Facebook\HttpClients\FacebookCurl; 

// init app with app id (APPID) and secret (SECRET)
FacebookSession::setDefaultApplication('XXXX', 'XXXXXXXXXXXX');

// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper( 'http://localhost/demo/demo2/demo2.php' );

try {

  $session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {
  // When Facebook returns an error
} catch( Exception $ex ) {
  // When validation fails or other local issues
}

// see if we have a session
if ( isset( $session ) ) {
  // graph api request for user data
  $request = new FacebookRequest( $session, 'GET', '/me' );
  $response = $request->execute();
  // get response
  $graphObject = $response->getGraphObject();

  // print data
  echo  print_r( $graphObject, 1 );
} else {
  // show login url
  echo '<a href="' . $helper->getLoginUrl() . '">Login</a>';
}

Question 4

You should be careful with all the paths!

I cannot see "autoload.php" in your file system. In my case I put the content of PHP SDK in the directory "fb", and use the paths (added DIR . into the "define" line in comparison with the example of https://developers.facebook.com/docs/php/gettingstarted/4.0.0)

define('FACEBOOK_SDK_V4_SRC_DIR', __DIR__ . '/fb/src/Facebook/');
require __DIR__ . '/fb/autoload.php';

echo FACEBOOK_SDK_V4_SRC_DIR; //to check if the paths are correct

use Facebook\FacebookSession;
use Facebook\FacebookRequest;
use Facebook\GraphUser;
use Facebook\FacebookRequestException;
<...>

It helped to avoid that kind of the error.

Question 5

No need to use require or include. I solved it. Just use following line at the top of your script:

define('FACEBOOK_SDK_V4_SRC_DIR', __DIR__ . '/facebook-php-sdk-v4/src/Facebook/');

and you are done.

Question 6

require_once 'Facebook/autoload.php';
$fb = new Facebook\Facebook([
  'app_id' => '{app_id}',
  'app_secret' => '{app_secret}',
  'default_graph_version' => 'v2.2',
]);

-just add this code .it works. enter your app_id and app_secret of your facebookAPP

Question 7

Rename all Facebook files and directories to lower case. Auto-loading on *nix automatically lowercases all files names.