Codeigniter facebook身份验证问题

Question

I am using sdk 5.5 for facebook login

After login on facebook page, on redirect url i am here is a check for user authentication.

if ($this->facebook->is_authenticated())

This line always returns false. I checked is_authenticated library function and found that $this->authenticate() is returning with empty access token.

All configurations and redirect url are correct. Below is facebook config code

$config['facebook_app_id']              = '******';
$config['facebook_app_secret']          = '***************';
$config['facebook_login_type']          = 'web';
$config['facebook_login_redirect_url']  = 'fb-login';
$config['facebook_logout_redirect_url'] = 'logout';
$config['facebook_permissions']         = array('public_profile', 'email');
$config['facebook_graph_version']       = 'v2.6';
$config['facebook_auth_on_load']        = TRUE;

Let me know if anyone need more information.