访问令牌不是在PHP PHP sdk5中生成的？

I am trying from last day to post on user timeline and for this I am using facebook PHP SDK5. I have two files one is fb-integration.php and second is login-callback.php

In fb-intergration.php - I have following code which is basically to check user login or not.

<?php
 session_start();
 require_once __DIR__ . '/src/Facebook/autoload.php';
 $fb = new Facebook\Facebook([
         'app_id' => '2426534xxxxxxxx',
         'app_secret' => 'dc36a92xxxxxxxxxxxxxxxxxxxxxxx',
         'default_graph_version' => 'v2.8',
]); 
 $helper = $fb->getRedirectLoginHelper();
 $permissions = ['email', 'publish_actions']; // optional
 $loginUrl = $helper->getLoginUrl('https://example.com/events/test/login-callback.php', $permissions);
 echo "<script>window.top.location.href='".$loginUrl."'</script>";
?>

In login-callback file - I have following script:

<?php
 session_start();
 require_once __DIR__ . '/src/Facebook/autoload.php';
 $fb = new Facebook\Facebook([
   'app_id' => '2426534xxxxxxxx',
   'app_secret' => 'dc36a92xxxxxxxxxxxxxxxxxxxxxxx',
   'default_graph_version' => 'v2.8',
 ]);
$helper = $fb->getCanvasHelper();
    try {
        if (isset($_SESSION['facebook_access_token'])) {
            $accessToken = $_SESSION['facebook_access_token'];
        } else {
            $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 ($accessToken) {
  echo 'Successfully logged in!';
}else{
    echo 'Sorry not generated';
}
?>

Every time, I am getting result Sorry not generated. Where am i doing wrong. why is not my accesToken generating?