How to log out user using facebook SDK?

Question

I am using a facebook log in for my web site using facebook php sdk.

What I noticed is the logout link doesn't do anything. After I logout, the user can still navigate the site. Here is my code in facebook.php:

<?php
    require 'src/facebook.php';
    $facebook = new Facebook(array(
           'appId'  => '*************',
           'secret' => '******************************',
    ));
    $user = $facebook->getUser();
    $loginUrl = $facebook->getLoginUrl();
    echo "<a href='$loginUrl'>login</a>";

    $logoutUrl = $facebook->getLogoutUrl();
    echo $loginUrl; 
    if($user){
        session_start() ; 
        $_SESSION['user_info'] = $user; 
        $_SESSION['user_pro']= $facebook->api('/me');
        print_r($_SESSION);
    }
    else{
        echo 'not logged in '; 
    }

    echo "<a href='example.com/logout.php'>log out </a>"
?>

This code works fine on log in. The log out link should destroy the session. Here is the header of the page:

<?php 
    print_r($_SESSION) ; 
    header('example.com') ; 
?>

The problem with my logout.php page is it doesn't detect the session at all. I don't know if this is a facebook api problem or my php problem.

How do you log the user out using the facebook SDK?

Answer 1

You can logout from your site as well as from facebook as follow by providing your site url to next parameter and destroying session

$token = $facebook->getAccessToken();
$url = 'https://www.facebook.com/logout.php?next=' . YOUR_SITE_URL .
  '&access_token='.$token;
session_destroy();
header('Location: '.$url);

You also have to log them out of your website AND you have to prevent your website from automatically remembering your user and re-logging them in immediately.

Disable the code that auto-logs in your user and try to logout again. Destroying the session will not prevent your site from creating a brand new valid session for the remembered user.

Answer 2

Here is the PHP logout code for my website that logs a user in and out with facebook. You don't have to destroy the session to logout the user, all you technically have to do is signal to your own website that this particular session may not be used to let the user in.

logout.php:

<?php
    require_once("facebook-php-sdk-6c82b3f/src/facebook.php");
    $config = array();
    $config['appId'] = '2911111111146';
    $config['secret'] = 'a6eaaaaaaaaaaaaaaaaaaaaaaaaaad1a';
    $config['fileUpload'] = false;
    $facebook = new Facebook($config);
    $logouturl = $facebook->getLogoutUrl();
    $_SESSION['user_facebook_email'] = "";
    $_SESSION['ask_user_to_login'] = true;
    header("Location: showquestions.php");
?>

index.php:

<?php
    require_once("facebook-php-sdk-6c82b3f/src/facebook.php");

    $config = array();
    $config['appId'] = '2911111111146';
    $config['secret'] = 'a6eaaaaaaaaaaaaaaaaaaaaaaaaaad1a';
    $config['fileUpload'] = false;

    $facebook = new Facebook($config);
    $userId = $facebook->getUser();
    if ($_SESSION['ask_user_to_login'] == true || $userId == 0){
      $loginUrl = $facebook->getLoginUrl();
      $_SESSION['ask_user_to_login'] = false;
      echo "<button type='button' onClick=\"window.location='$loginUrl'\">" .
           "<img src='picture.gif' alt='Login with facebook'/>" .
           "</button>";
      exit;
    }
    else
    {
      $userInfo = $facebook->api('/' + $userId);
      session_cache_expire (150000);  //set the cache expire to 15000 minutes
      $_SESSION['user_facebook_email'] = $userInfo['email'];
      $_SESSION['facebook'] = $facebook;      
      header("Location: showquestions.php");
    }
    $userInfo = $facebook->api('/' + $userId);
    echo "Welcome" . $userInfo['email'];
?>

login.php:

<?php
    session_start();        
    if (isset($_SESSION['user_facebook_email']) !== true || 
        $_SESSION['user_facebook_email'] == "")
    {
        header("Location: index.php");
        exit;
    }
?>

Then in every php file you want to prevent access without a logged-in user, put this at the top:

<?php
    require("log2.php");
?>

With this code, the user is logged in automatically, and if they invoke the logout code, the site will not let them in until they login again.