Facebook SDK v4 $session not working - php

Question

Hi I am using facebook SDKv4 for facebook login on my website
After authentication of app the user is not login into my site neither the details are saving to the database and after authentication i can see the login button
<?php
session_start();
require_once( 'Facebook/HttpClients/FacebookHttpable.php' );
require_once( 'Facebook/HttpClients/FacebookCurl.php' );
require_once( 'Facebook/HttpClients/FacebookCurlHttpClient.php' );
require_once( 'Facebook/Entities/AccessToken.php' );
require_once( 'Facebook/Entities/SignedRequest.php' );
require_once( 'Facebook/FacebookSession.php' );
require_once( 'Facebook/FacebookRedirectLoginHelper.php' );
require_once( 'Facebook/FacebookRequest.php' );
require_once( 'Facebook/FacebookResponse.php' );
require_once( 'Facebook/FacebookSDKException.php' );
require_once( 'Facebook/FacebookRequestException.php' );
require_once( 'Facebook/FacebookAuthorizationException.php' );
require_once( 'Facebook/GraphObject.php' );
require_once( 'Facebook/GraphSessionInfo.php' );
use Facebook\Entities\AccessToken;
use Facebook\Entities\SignedRequest;
use Facebook\FacebookSession;
use Facebook\FacebookRedirectLoginHelper;
use Facebook\FacebookRequest;
use Facebook\FacebookResponse;
use Facebook\FacebookSDKException;
use Facebook\FacebookRequestException;
use Facebook\FacebookAuthorizationException;
use Facebook\GraphObject;
use Facebook\GraphSessionInfo;
// init app with app id (APPID) and secret (SECRET)
FacebookSession::setDefaultApplication('APPID','SECRET');
// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper( 'example.com' );
/***************************************************session handinling*********************************************/
// see if a existing session exists
if ( isset( $_SESSION ) && isset( $_SESSION['fb_token'] ) ) {
// create new session from saved access_token
//$_SESSION['fb_token'] = $session->getToken();
// create a session using saved token or the new one we generated at login
// $session = new FacebookSession( $session->getToken() );
$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();
} catch( FacebookRequestException $ex ) {
// When Facebook returns an error
// handle this better in production code
print_r( $ex );
} catch( Exception $ex ) {
// When validation fails or other local issues
// handle this better in production code
print_r( $ex );
}
}
/*****************************************************session handling*****************************************/
try {
$session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {
// When Facebook returns an error
} catch( Exception $ex ) {
// When validation fails or other local issues
}
// see if we have a session
if ( isset( $session ) ) {
// graph api request for user data
$_SESSION['fb_token'] = $session->getToken();
// create a session using saved token or the new one we generated at login
$session = new FacebookSession( $session->getToken() );
$request = new FacebookRequest( $session, 'GET', '/me' );
$response = $request->execute();
// get response
$graphObject = $response->getGraphObject();
$fuid= $graphObject->getProperty('id'); // To Get Facebook ID
$fname=$graphObject->getProperty('first_name'); // To Get Facebook full name
$lname=$graphObject->getProperty('last_name'); // To Get Facebook full name
$femail=$graphObject->getProperty('email');
$gender=$graphObject->getProperty('gender');
CODE TO SAVE DETAILS TO DB
/*************************************************************post on users walll******************************************************/
// Publish to User’s Timeline
$request = ( new FacebookRequest( $session, 'POST', '/me/feed', array(
'message' => 'Only TEXT Goes Here'
) ) )->execute();
// Get response as an array, returns ID of post
$response = $request->getGraphObject()->asArray();
print_r( $response );
// Graph API to publish to timeline with additional parameters
$request = ( new FacebookRequest( $session, 'POST', '/me/feed', array(
'name' => 'Name',
'caption' => 'CAtion',
'link' => 'example.com',
'message' => 'msg'
) ) )->execute();
// Get response as an array, returns ID of post
$response = $request->getGraphObject()->asArray();
/****************************************************************post on users wall************************************************/
redirect($site_url);
//echo print_r( $graphObject, 1 );
} else {
// show login url
$permissions = ['email', 'user_likes','publish_actions'];
echo '<img src="'.$img_cdn.'fbcon.png">';
}
?>
Can any one tell where i am doing wrong ?