How to use Facebook PHP SDK v5.0 in wordpress plugin?

I want to develop voting plugin for WordPress. Votes can be given using only facebook login. My folder structure as follow,

plugin-
 |-admin
 |-includes
 |---Facebook/
 |---autoload.php
 |---loadsdk.php
 |-plugin.php

I tried, include( MY_PLUGIN_PATH . 'includes/loadsdk.php'); in plugin.php.

<?php

session_start();

define("FB_APP_ID", "345345345345345");
define("FB_APP_SECRET", "dsfgdsfgsdfgsdfg");
define("REDIRECT_URL", "https://myweb.com/folder/");
define("FB_PERMISSIONS",'email');

require 'autoload.php';

use Facebook\FacebookSession;
use Facebook\FacebookRequest;
use Facebook\FacebookResponse;

use Facebook\GraphNodes\GraphUser;
use Facebook\Entities\SignedRequest;
use Facebook\Exceptions\FacebookRequestException;
use Facebook\Helpers\FacebookRedirectLoginHelper;

// init app with app id and secret
FacebookSession::setDefaultApplication(FB_APP_ID, FB_APP_SECRET);

// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper(REDIRECT_URL);

// see if a existing session exists
if (isset($_SESSION) && isset($_SESSION['fb_token'])) {
	// create new session from saved access_token
	$session = new FacebookSession($_SESSION['fb_token']);

	// validate the access_token to make sure it's still valid
	try {
		if (!$session -> validate()) {
			$session = null;
		}
	} catch ( Exception $e ) {
		$session = null;
	}
}

if (!isset($session) || $session === null) {
	// no session exists
	try {
		$session = $helper -> getSessionFromRedirect();
	} catch( FacebookRequestException $ex ) {
	} catch( Exception $ex ) {
	}
}

// see if we have a session

if (isset($session)) {
	// save the session
	$_SESSION['fb_token'] = $session -> getToken();

	// create a session using saved token or the new one we generated at login
	$session = new FacebookSession($session -> getToken());

	// graph api request for user data
	$request = new FacebookRequest($session, 'GET', '/me?fields=first_name,last_name,email');
	$response = $request -> execute();

	// get response
	$graphObject = $response -> getGraphObject() -> asArray();

	print_r($graphObject);

	// print logout url using session and redirect_uri (logout.php page should destroy the session)
	$logoutURL = $helper -> getLogoutUrl($session, CANVAS);

} else {

	// show login url
	echo $loginURL = '<a href="' . $helper -> getLoginUrl(array(FB_PERMISSIONS)) . '">Login with facebook</a>';

}
?>

I can see bottom login-url on web site.though i click on it redirect with code as response. I think session is not creating.