The callback page for get access token from Facebook

callback.php

<?php
require_once __DIR__ . '/vendor/autoload.php';

session_start(); 

$fbData = array(
    'app_id' => '{APP ID}',
    'app_secret' => '{APP SECRET}',
    'default_graph_version' => 'v2.2'
);
 
$fb = new Facebook\Facebook($fbData);
 
$helper = $fb->getRedirectLoginHelper();

try {
  $accessToken = $helper->getAccessToken();

  // OAuth 2.0 client handler
  $oAuth2Client = $fb->getOAuth2Client();
 
  // Exchanges a short-lived access token for a long-lived one
  $accessToken = $oAuth2Client->getLongLivedAccessToken($accessToken);

  echo "Access Token: " . $accessToken;

} catch(Facebook\Exceptions\FacebookResponseException $e) {
  // When Graph returns an error
  echo 'Graph returned an error: ' . $e->getMessage();
  exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
  // When validation fails or other local issues
  echo 'Facebook SDK returned an error: ' . $e->getMessage();
  exit;
}