使用facebook js sdk和php sdk

I believe I have read almost all if not all the related posts, but I still can't find my answer.

On client side I get the user to login and send user id and access token to server. Then server use facebook->getUser() to check if the sent userid really belong to him, if so I add him to my database and register him. similar to this question

Facebook Login: How to combine JavaScript with PHP SDK?

except facebook->getUser() keep returning 0. I have look through A LOT of questions related to that too, but I just cant find my answer. No problem with client side though, user can log in and give authority, it generate the correct userid and a token.

I am currently using localhost to test everything.

php:

<?php
require 'facebook/src/facebook.php';

$uid = $_POST['userid'];
$token = $_POST['accessToken'];


$facebook = new Facebook(array(
  'appId'  => 'XXXXXXXXXXXXXXXXX',
  'secret' => 'XXXXXXXXXXXXXXXXXXXXXXXX',
));

// Get User ID
$user = $facebook->getUser();

if($user==$uid){
//codes here

My question is similar to the following posts but it didnt get any answer Facebook API: Login using JavaScript SDK then checking login state with PHP

additional info: the reason why I want to use PHP sdk too is because, I dont want people to use someone else's fb userID to send request to my server 'pretending' to be another person

You receive the FB Token from your FB Javascript SDK. Are you setting the FB Token in PHP first with the one you received from Javascript?

$facebook->setAccessToken($new_access_token);

before calling?

$facebook->getUser()