مشكلة فى facebook login

ارغب في عمل تسجيل دخول ب فيس بوك فى موقعي.. هناك رسالة خطأ تظهر وهي

undefined variable  $facebook_login_url;

<!--facebook login -->
                <!--<div class="div-input-login1">
                  <a href="<?php //echo $facebook_login_url; ?>"><button class="submit btnFB" type="submit"  class="btn btn-primary " ><?php echo $lang['fbLogin']?></button></a>
                </div>-->

<?php
ob_start();

  require_once 'Facebook/autoload.php'; 

  // init configuration

if (!session_id())
{
    session_start();
}

// Call Facebook API

$facebook = new \Facebook\Facebook([
  'app_id'      => '898876554420136',
  'app_secret'     => 'ae49ju76t549ae4c4be43gh78u2h9fa12',
  'default_graph_version'  => 'v2.10'
]);



<?php
include 'facebookConfig.php';

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($accessToken)) {
  if ($helper->getError()) {
    header('HTTP/1.0 401 Unauthorized');
    echo "Error: " . $helper->getError() . "\n";
    echo "Error Code: " . $helper->getErrorCode() . "\n";
    echo "Error Reason: " . $helper->getErrorReason() . "\n";
    echo "Error Description: " . $helper->getErrorDescription() . "\n";
  } else {
    header('HTTP/1.0 400 Bad Request');
    echo 'Bad request';
  }
  exit;
}
 
if(!$accessToken->isLongLived()){
  // Exchanges a short-lived access token for a long-lived one
  try {
    $accessToken = $oAuth2Client->getLongLivedAccessToken($accessToken);
  } catch (Facebook\Exceptions\FacebookSDKException $e) {
    echo "<p>Error getting long-lived access token: " . $e->getMessage() . "</p>\n\n";
    exit;
  }
}
 
$fb->setDefaultAccessToken($accessToken);
 
# These will fall back to the default access token
$res    =   $fb->get('/me',$accessToken->getValue());
$fbUser =   $res->getDecodedBody();
 
 
$resImg     =   $fb->get('/me/picture?type=large&redirect=false',$accessToken->getValue());
$picture    =   $resImg->getGraphObject();
 
 
$_SESSION['fbUserId']       =   $fbUser['id'];
$_SESSION['fbUserName']     =   $fbUser['name'];
$_SESSION['fbAccessToken']  =   $accessToken->getValue();
 
header('Location: welcome.php');
exit;
?>

     
ob_end_flush();