Failed to get access token from facebook?

require_once "/vendor/autoload.php";

  $fb = new Facebook\Facebook([
    "app_id" => "xxx",
    "app_secret" => "xxx", 
    "default_graph_version" => "v2.8",
    ]);

  $helper = $fb->getRedirectLoginHelper();

  $permissions = ["email"];
  $loginUrl = $helper->getLoginUrl("https://xxx.xxx.com/fb-sdk/fb-callback.php", $permissions);

  echo "<a href="" . htmlspecialchars($loginUrl) . "">Log in with Facebook!</a>";

fb-callback.php:

require_once "/vendor/autoload.php";

$fb = new \Facebook\Facebook([
  "app_id" => "xxx",
  "app_secret" => "xxx",
  "default_graph_version" => "v2.8",

]);

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

try {
  $accessToken = $helper->getAccessToken();
  // 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?fields=id,name,email", $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;
}

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

he will get stuck in $accessToken = $helper- > getAccessToken (); and will not run.
print out any news
and report:
Facebook SDK returned an error: Cross-site request forgery validation failed.. Required param "state" missing from persistent data.

he replied to my fb-callback.php with these two key

fb-callback.php?code=***&state=***-sharp_=_

I think it"s token, so I try to replace $accessToken
, but it"s still wrong to say that token format is not right for
. What"s wrong with it?
I"m sure login is OK.
when I enter login.php and press Login, I will go to facebook, and ask if I agree. I press yes and go straight to fb-callback.php, but there is no token in my report.

I follow this: https://github.com/facebook/p.

// fb-callback.phpsession if (!session_id()) { session_start(); } // $helper = $fb->getRedirectLoginHelper(); if (isset($_GET['state'])) { $helper->getPersistentDataHandler()->set('state', $_GET['state']); }