Native PHP session variables are automatically destroy after Facebbok PHP login

I am using facebook php sdk 4 for my application. I have add few data on $_SESSION. But those session variables are getting erased after login using FacebookRedirectLoginHelper::getLoginUrl here is code below:

session_start();
session_regenerate_id(true);


define( 'ROOT', dirname( __FILE__ ) . '/' );


require_once( ROOT . 'facebook-php-sdk-v4-4.0-dev/autoload.php' );

use Facebook\FacebookSession;
use Facebook\FacebookRedirectLoginHelper;

$app_id = 'MY ID';
$app_secret = 'MY SEC';
$redirect_uri = 'http://myurl.com/a1.php';

// Requested permissions for the app - optional
$permissions = array(
'email',
'user_location',
'user_birthday',
'publish_actions'
);



// Initialize the SDK
FacebookSession::setDefaultApplication( $app_id, $app_secret );

$helper = new FacebookRedirectLoginHelper( $redirect_uri );


if ( isset( $_SESSION ) && isset( $_SESSION['fb_token'] ) )
{

$session = new FacebookSession( $_SESSION['fb_token'] );

try {
if ( ! $session->validate() ) {
$session = null;
}
} catch ( Exception $e ) {
// Catch any exceptions
$session = null;
}
} else {

try {
$session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {


} catch( Exception $ex ) {


echo $ex->message;
}
}


if ( isset( $session ) ) {


$_SESSION['fb_token'] = $session->getToken();


$session = new FacebookSession( $session->getToken() );





}

else {
// No session

$_SESSION["V_ActualData_Message"]="My Message"; **// I am not getting this one on $redirect_uri here http://myurl.com/a1.php**

$loginUrl = $helper->getLoginUrl( $permissions );
header('Location: ' . $loginUrl);
exit;
//echo '<a href="' . $loginUrl . '">Log in with FaceBook</a>';
}