使用 PHP SDK 进行身份验证后，Facebook 会话为 NULL

我可以使用我的 Facebook 应用程序登录，但我得到的会话始终为 NULL。 这是我的整个 PHP 代码：

<?php


session_start(); //Session should always be active

  $app_id               = 'mySexyAppID';  //DEV Name: PHP_SDK-Prev-FB\0.0.2b4
  $app_secret       = 'someS3cr3tStuff';
  $required_scope   = 'public_profile, publish_actions,publish_stream,manage_pages,user_groups'; //Permissions required
  $redirect_url         = 'http://localhost/fb/facebook-sdk-v4-sample-files/index2.php'; //FB redirects to this page with a code

  require_once __DIR__ . "/facebook-php-sdk-v4-4.0-dev/autoload.php"; //include autoload from SDK folder

  //define output vars
  $responseMembers_String  = '0';

  //import required class to the current scope
  use Facebook\FacebookSession;
  use Facebook\FacebookRequest;
  use Facebook\GraphUser;
  use Facebook\FacebookRedirectLoginHelper;

  FacebookSession::setDefaultApplication($app_id , $app_secret);
  $helper = new FacebookRedirectLoginHelper($redirect_url);

  /**************************************************************/
  // see if a existing session exists
  if ( isset( $_SESSION ) && isset( $_SESSION['fb_token'] ) ) {
    // create new session from saved access_token
    $session = new FacebookSession( $_SESSION['fb_token'] );

    // validate the access_token to make sure it's still valid
    try {
      if ( !$session->validate() ) {
        $session = null;
      }
    } catch ( Exception $e ) {
      // catch any exceptions
      $session = null;
    }
  }  

  if ( !isset( $session ) || $session === null ) {
    // no session exists
    try {
      $session = $helper->getSessionFromRedirect();
      $_SESSION["FBSession"] = $session;
    } catch( FacebookRequestException $ex ) {
      print_r( $ex->getMessage());
    } catch( Exception $ex ) {
      print_r( $ex->getMessage());
    }

  }

  // see if we have a session
  if ( isset( $session ) ) {
      $_SESSION["SessionSetText"] = "Session set with success!";
  }
  /**************************************************************/



  //if user wants to log out
  if(isset($_GET["log-out"]) && $_GET["log-out"]==1){
      unset($_SESSION["fb_user_details"]);
      //session ver is set, redirect user 
      header("location: ". $redirect_url);
  }

  //Test normal login / logout with session

  if ($session){ //if we have the FB session
      //get user data
      $user_profile = (new FacebookRequest($session, 'GET', '/me'))->execute()->getGraphObject(GraphUser::className());
      $user_accounts = (new FacebookRequest($session, 'GET', '/me/accounts'))->execute()->getGraphObject(GraphUser::className());
      $user_groups = (new FacebookRequest($session, 'GET', '/me/groups'))->execute()->getGraphObject(GraphUser::className());

      //save session var as array
      $_SESSION["fb_user_details"] = $user_profile->asArray(); 
      $_SESSION["accounts"] = $user_accounts->asArray();
      $_SESSION["groups"] = $user_groups->asArray();

      $user_id = ( isset( $_SESSION["fb_user_details"]["id"] ) )? $_SESSION["fb_user_details"]["id"] : "";
      $user_name = ( isset( $_SESSION["fb_user_details"]["name"] ) )? $_SESSION["fb_user_details"]["name"] : "";
      $user_email = ( isset( $_SESSION["fb_user_details"]["email"] ) )? $_SESSION["fb_user_details"]["email"] : "";

      //session var is set, redirect user 
      header("location: ". $redirect_url);

  }

  if(isset($_POST["group_submit"]))
      {
          //user wants informations about a group where he is administrator
          $userGroupId = $_POST["usergroups"];
          //$group_url = '/'.$userGroupId.'/members';

          //request members via facebook graph api  
          /**************************************************/
          $requestMembers = new FacebookRequest($session, 'GET', '/'.$userGroupId.'/members');
          $requestMembers_response = $requestMembers->execute()->getGraphObject()->asArray();

          /**************************************************/

          $responseMembers_String =  $requestMembers_response;
          $_SESSION["GroupMembers"] = $responseMembers_String;
      }
?>

<html>
  <head>
    <meta charset="utf-8" />
    <title>Demo StackOverflow</title>
  </head>

  <body>
  <form id="form" name="form" method="POST" action="index2.php">
          <?php

              if(isset($_SESSION["fb_user_details"]))
              {
                  print 'Hi '.$_SESSION["fb_user_details"]["name"].' you are logged in! [ <a href="?log-out=1">log-out</a> ] ';
              }
              else{
                  //display login url 
                  $login_url = $helper->getLoginUrl( array( 'scope' => $required_scope ) );
                  echo '<a href="'.$login_url.'">Connect with Facebook</a>';
              };
              ?>
          <br />
          <br />
                  <?php
                  if(isset($_SESSION["fb_user_details"]))
                  {
                      print 'Some infos about the current user ['.$_SESSION["fb_user_details"]["first_name"].']: <pre>';
                      print_r($_SESSION["fb_user_details"]);
                      print '</pre>';   
                  };
                  ?>

          <br />
          <select name="usergroups" id="ugroups">
                      <?php 
                          foreach ($_SESSION["groups"]["data"] as $group) {
                              //only fetch groups where user is admin!
                              if($group->administrator)
                              {
                                  echo '<option value="'.$group->id.'">'.$group->name.'</option>';  

                              }
                          } ;
                      ?>
          </select>
          <br />
              <button type="submit" class="button" id="group_submit" name="group_submit">Get some informations!</button><br />
          <label id="GroupMembers" name="GroupMembers"><?php if (!(isset($_SESSION["GroupMembers"]))){print 'not set now!';} elseif(isset($_SESSION["GroupMembers"])) {print 'the selected group has '.$responseMembers_String.' Members';} ?></label><br />
          <label id="lInfos" name="lInfos"><?php if (!(isset($_SESSION["FBSession"]))){print '$session not set!!';} elseif(isset($_SESSION["FBSession"])) {print 'Session set!';} ?></label>
      </form>
  </body>
</html>

当我调用图形函数时（请求组成员：$requestMembers = new FacebookRequest($session, 'GET', '/'.$userSelectedGroupID.'/members');）我收到以下错误：

Catchable fatal error: Argument 1 passed to Facebook\FacebookRequest::__construct() must be an instance of Facebook\FacebookSession, null given

我已经更新了问题并添加了包含所有代码的完整示例。 我意识到，我可以在任何地方进行会话，因为我可以选择用户组。