validating-access-token.php

<?php
session_start();
require_once __DIR__ . '/src/Facebook/autoload.php'; // download official fb sdk for php @ https://github.com/facebook/php-graph-sdk

$fb = new Facebook\Facebook([
  'app_id' => 'APP_ID',
  'app_secret' => 'APP_SECRET',
  'default_graph_version' => 'v2.12',
  ]);

$helper = $fb->getCanvasHelper();
$permissions = ['email']; // optional

try {
	if (isset($_SESSION['facebook_access_token'])) {
	$accessToken = $_SESSION['facebook_access_token'];
	} else {
  		$accessToken = $helper->getAccessToken();
	}
} 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;
 }

if (isset($accessToken)) {
	if (isset($_SESSION['facebook_access_token'])) {
		$fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
	} else {
		$_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;

		$fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
	}

	// validating the access token
	try {
		$request = $fb->get('/me');
	} catch(Facebook\Exceptions\FacebookResponseException $e) {
		// When Graph returns an error
		if ($e->getCode() == 190) {
			unset($_SESSION['facebook_access_token']);
			$helper = $fb->getRedirectLoginHelper();
			$loginUrl = $helper->getLoginUrl('https://apps.facebook.com/APP_NAMESPACE/', $permissions);
			echo "<script>window.top.location.href='".$loginUrl."'</script>";
		}
		exit;
	} catch(Facebook\Exceptions\FacebookSDKException $e) {
		// When validation fails or other local issues
		echo 'Facebook SDK returned an error: ' . $e->getMessage();
		exit;
	}

	// getting basic info about logged-in user
	try {
		$request = $fb->get('/me?fields=name,email');
		$profile = $request->getGraphNode()->asArray();
	} 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;
	}

	echo $profile['name'];

  	// Now you can redirect to another page and use the access token from $_SESSION['facebook_access_token']
} else {
	$helper = $fb->getRedirectLoginHelper();
	$loginUrl = $helper->getLoginUrl('https://apps.facebook.com/APP_NAMESPACE/', $permissions);
	echo "<script>window.top.location.href='".$loginUrl."'</script>";
}