Access Token not generated in facebook PHP sdk5? - php

Question

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?