Session not active, could not store state Facebook SDK

It was session start problem, so you must start your session properly at the start of the page like this....

define('FACEBOOK_SDK_V4_SRC_DIR','../Vendor/fb/src/Facebook/');
require_once("../Vendor/fb/autoload.php");
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\GraphUser;
use Facebook\GraphSessionInfo;
if (session_status() == PHP_SESSION_NONE) {
    session_start();
}

FacebookSession::setDefaultApplication(FACEBOOK_APP_ID, FACEBOOK_APP_SECRET);
$helper = new FacebookRedirectLoginHelper(FACEBOOK_REDIRECT_URI);
$this->set('fb_url', $helper->getLoginUrl());

You may also refer following tutorial facebook oAuth login using php.

Facebook OAuth 2 Login Using PHP

If you start a session that's already started you will get php notice error that's session already started. so It always better idea to check a session before you are going to start, so starting your php session using following php script is always safe.

   if (session_status() == PHP_SESSION_NONE) {
       session_start(); 
   } 

If you are using CakePHP use session_start() at the start of the controller function .

public function login()
{
   if (session_status() == PHP_SESSION_NONE) {
    session_start(); 
   } 
   FacebookSession::setDefaultApplication(FACEBOOK_APP_ID, FACEBOOK_APP_SECRET);
   $helper = new FacebookRedirectLoginHelper(FACEBOOK_REDIRECT_URI);
   $this->set('fb_url', $helper->getLoginUrl());
}