call back

login.php
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<?php
require_once 'Facebook/autoload.php';

session_start();
$fb = new Facebook\Facebook([
  'app_id' => 'id', // Replace {app-id} with your app id
  'app_secret' => 'secret',
  'default_graph_version' => 'v3.2',
  ]);

$helper = $fb->getRedirectLoginHelper();

$permissions = ['email,manage_pages, pages_show_list, pages_messaging, public_profile']; // Optional permissions
$loginUrl = $helper->getLoginUrl('https://localhost/fb/fb-callback.php', $permissions);

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

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Facebook-callback.php

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<?php
require_once 'Facebook/autoload.php';


session_start();
$accessToken = 'token';
$fb = new Facebook\Facebook([
  'app_id' => 'id', // Replace {app-id} with your app id
  'app_secret' => 'secret',
  'default_graph_version' => 'v3.2',
  ]);

$helper = $fb->getRedirectLoginHelper();
if (isset($_GET['state'])){
  $helper ->getPersistentDataHandler()->set('state', $_GET['state']);
}

try {
  // Returns a `FacebookFacebookResponse` object
  $response = $fb->get(
    '/<post-id>/comments/?fields=from,created_time,message&limit=100',
    $accessToken
  );
} catch(FacebookExceptionsFacebookResponseException $e) {
  echo 'Graph returned an error: ' . $e->getMessage();
  exit;
} catch(FacebookExceptionsFacebookSDKException $e) {
  echo 'Facebook SDK returned an error: ' . $e->getMessage();
  exit;
}
$graphEdge = $response->getGraphEdge();
echo "<pre>";
echo $graphEdge;
?>