Using Facebook SDK for user authentication

<?php
//NOTE: This code wasn't tested but it should work fine since i almost copied and pasted everything from my previous work i've tested
//Hope it guides you well enough.
//
//Defining some constants i need, replace with yours
define('HOSTNAME', 'http://localhost/sample');
define('CURRENT_PAGE', HOSTNAME . '/my_login_page.php');


//THEN HERE WE GO!
//
//
//Step 1
//Create a Facebook APP on Facebook developer page
//Make it public and copy your APP ID and APP SECRET
//Add your HOSTNAME to the urls allowed to access the app
//Download facebook sdk and extract to current folder
//You can browse how to make it public and how to add urls if you don get it at first hand, can't remember the steps right now
//Step 2
//Start session, Facebook API needs it and you might need it too.
//It might be important you understand how PHP session works
session_start();

//load Facebook api
require_once __DIR__ . '/path-to-downloaded-sdk/src/Facebook/autoload.php';

//Create Facebook object
$fb = new Facebook\Facebook([
    'app_id' => 'YOUR_APP_ID',
    'app_secret' => 'YOUR_APP_SECRET',
    'default_graph_version' => 'v2.2',
        ]);
//you'll need to repeat Step 2 on any other you have to use this Facebook API
//
//
//Step 3
//Assuming this is the same page you want facebook to redirect user after authentication
//Check if this was a redirect from facebook
//Am using the 'token' variable to identify a request uniquely, so its what i check for.
//If the 'token' variable exists in the $_GET then i know its from Facebook and the user has visited this page before
//I also made a copy of the token in the $_SESSION to match with the one from Facebook to be sure the user is not trying to bypass this step

if (isset($_GET['token']) and isset($_SESSION['token'])) {
    //This is a redirect from Facebook
    //Compare tokens to avoid unauthorized user access
    if ($_GET['token'] === $_SESSION['token']) {

        $helper = $fb->getRedirectLoginHelper();
        try {
            $accessToken = $helper->getAccessToken();
            if (isset($accessToken)) {
                //User logged in and was sucessfully authenticated!
                //Save accessToken wherever you like, am saving to session
                //This access token is what you use to extract the info you requested for about this user from facebook,
                //as far the session created by facebook api allows you, you can reuse the accessToken anywhere
                $_SESSION['facebook_access_token'] = (string) $accessToken;

                //Extracting info
                // Sets the default fallback access token so we don't have to pass it to each request
                $fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
                try {
                    //Get fields you need
                    $response = $fb->get('/me?fields=id,first_name,last_name,picture,email');
                    $userNode = $response->getGraphUser();

                    $array = array();
                    $array['id'] = $userNode->getId();
                    $array['first_name'] = $userNode->getFirstName();
                    $array['last_name'] = $userNode->getLastName();
                    $pic = $userNode->getPicture();
                    if (!empty($pic)) {
                        $array['pic_url'] = $pic->getUrl();
                    }
                    $array['email'] = $userNode->getEmail();

                    //Now you have the information you want
                    //You might want to save them to database now then redirect the user to his profile or wherever
                    //And were done!!!!! phew!!
                } catch (Facebook\Exceptions\FacebookResponseException $e) {
                    // When Graph returns an error
                    $error = 'Graph returned an error: ' . $e->getMessage();
                } catch (Facebook\Exceptions\FacebookSDKException $e) {
                    // When validation fails or other local issues
                    $error = 'Facebook SDK returned an error: ' . $e->getMessage();
                }
            }
        } catch (Facebook\Exceptions\FacebookResponseException $e) {
            // When Graph returns an error
            $error = 'Graph returned an error: ' . $e->getMessage();
        } catch (Facebook\Exceptions\FacebookSDKException $e) {
            // When validation fails or other local issues
            $error = 'Facebook SDK returned an error: ' . $e->getMessage();
        }
    } else {
        //User may have tried to bypass authenticaton
        //Do something and show him who's Boss :)
        //Erm.. It's best to just notify him authentication failed
    }
} else {
    //This is probably User's first visit and not from Facebook

    $helper = $fb->getRedirectLoginHelper();
    $permissions = ['email', 'user_likes']; // optional, check developer page for more permissions you might need
    //Generate token for unique identification of request
    $token = md5(uniqid());
    //token saved to session
    $_SESSION['token'] = $token;

    //Generate login url, this url redirects the user to facebook and does all the authentication
    $loginUrl = $helper->getLoginUrl(CURRENT_PAGE . "?token=$token", $permissions);
}
?>

<?php
if (isset($error)) {
    echo $error;
}
?>
<div style="padding: 20px 20px 20px 20px; text-align: center">
    <a href="<?= $loginUrl ?>" style="padding: 5px 5px 5px 5px; background: blue;">Log in with Facebook</a>
</div>