$fb = new Facebook\Facebook([
  'app_id' => '{app-id}',
  'app_secret' => '{app-secret}',
  //'default_access_token' => '{access-token}', // optional
]);

// Use one of the helper classes to get a Facebook\AccessToken entity.
//   $helper = $fb->getRedirectLoginHelper();
//   $helper = $fb->getJavaScriptHelper();
//   $helper = $fb->getCanvasHelper();
//   $helper = $fb->getPageTabHelper();

try {
  // Get the Facebook\GraphNodes\GraphUser object for the current user.
  // If you provided a 'default_access_token', the '{access-token}' is optional.
  $response = $fb->get('/me', '{access-token}');
} 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;
}

$me = $response->getGraphUser();
echo 'Logged in as ' . $me->getName();