session_start();
require_once __DIR__ . '/src/autoload.php'; // download official fb sdk for php @ https://github.com/facebook/php-graph-sdk
$fb = new FacebookFacebook([
  'app_id' => '2030958897177886',
  'app_secret' => '49bef31b7b89d4a6c0da9a5c797f8892',
  'default_graph_version' => 'v2.11',
  ]);
$helper = $fb->getRedirectLoginHelper();
$permissions = ['email']; // optional

// print_r($helper);
// exit();

try {
    if (isset($_SESSION['facebook_access_token'])) {
        $accessToken = $_SESSION['facebook_access_token'];
    } else {
        $accessToken = $helper->getAccessToken();
    }
} catch(FacebookExceptionsFacebookResponseException $e) {
    // When Graph returns an error
    echo 'Graph returned an error: ' . $e->getMessage();
    exit;
} catch(FacebookExceptionsFacebookSDKException $e) {
    // When validation fails or other local issues
    echo 'Facebook SDK returned an error: ' . $e->getMessage();
    exit;
}

if (isset($accessToken)) {
    if (isset($_SESSION['facebook_access_token'])) {
        $fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
    } else {
        // getting short-lived access token
        $_SESSION['facebook_access_token'] = (string) $accessToken;
        // OAuth 2.0 client handler
        $oAuth2Client = $fb->getOAuth2Client();
        // Exchanges a short-lived access token for a long-lived one
        $longLivedAccessToken = $oAuth2Client->getLongLivedAccessToken($_SESSION['facebook_access_token']);
        $_SESSION['facebook_access_token'] = (string) $longLivedAccessToken;
        // setting default access token to be used in script
        $fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
    }
    // redirect the user back to the same page if it has "code" GET variable
    if (isset($_GET['code'])) {
        header('Location: ./');
    }
    // getting basic info about user
    try {
        $profile_request = $fb->get('/me?fields=name,first_name,last_name,email,picture');
        $user = $profile_request->getGraphNode()->asArray();
    } catch(FacebookExceptionsFacebookResponseException $e) {
        // When Graph returns an error
        echo 'Graph returned an error: ' . $e->getMessage();
        session_destroy();
        // redirecting user back to app login page
        header("Location: ./");
        exit;
    } catch(FacebookExceptionsFacebookSDKException $e) {
        // When validation fails or other local issues
        echo 'Facebook SDK returned an error: ' . $e->getMessage();
        exit;
    }