//libraries/Facebook.php

<?php

define('FACEBOOK_SDK_DIR', 'facebook-php-sdk/src/Facebook/');

class Facebook{
	function __construct(){
		require __DIR__ . '/facebook-php-sdk/autoload.php';
	}
}

//controllers/Welcome.php

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

use Facebook\FacebookSession;
use Facebook\FacebookRequest;
use Facebook\GraphUser;
use Facebook\FacebookRequestException;
use Facebook\FacebookRedirectLoginHelper;

class Welcome extends CI_Controller {
	public $user = "";
	
	public function __construct() {
		parent::__construct();
		session_start();
		$this->load->library('facebook');
		FacebookSession::setDefaultApplication('___', '_______________');
	}

	public function oauth_login() {
		try {
			$session = $this->_get_session(base_url('welcome/oauth_login'));
			if($session){
				$me = (new FacebookRequest($session, 'GET', '/me'))->execute()->getGraphObject(GraphUser::className());
				echo $me->getName();
			}
		}
		catch(FacebookRequestException $e) {
			echo $e->getMessage();
		}
		catch(\Exception $e) {
			echo $e->getMessage();
		}
	}
	
	private function _get_session($url) {
		$helper = new FacebookRedirectLoginHelper($url);
		try {
			$session = $helper->getSessionFromRedirect();
		}
		catch(FacebookRequestException $e) {
			echo $e->getMessage();
		}
		catch(\Exception $e) {
			echo $e->getMessage();
		}

		if(isset($session)){
			return $session;
		} else{
			echo '<a href="', $helper->getLoginUrl(), '">Login Facebook</a>';
			return false;
		}
	}
}