SDK错误“状态”URL的参数和会话不匹配

我使用的是来自Facebook的代码

https://developers.facebook.com/docs/php/gettingstarted/5.0.0

但现在它显示Facebook SDK returned an error: Cross-site request forgery validation failed. The "state" param from the URL and session do not match.

我不明白怎么了

我的登录回调页面代码

session_start();
require_once __DIR__ . '/facebook-php-sdk-v4-5.0-dev/src/Facebook/autoload.php';
$fb = new Facebook'Facebook([
  'app_id' => 'xxxx',
  'app_secret' => 'xxxxxxxx',
  'default_graph_version' => 'v2.2',
  ]);
$helper = $fb->getRedirectLoginHelper();

try {
  $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)) {
  // Logged in!
  $_SESSION['facebook_access_token'] = (string) $accessToken;
  // Now you can redirect to another page and use the
  // access token from $_SESSION['facebook_access_token']
  $_SESSION['facebook_access_token'];
}

$_SESSION['FBRLH_state']=$_GET['state'];