Dropbox-sdk php

Question

index.php

<?php
include_once __DIR__.'/app/start.php';
include_once __DIR__.'/app/dropbox_auth.php';
?>

start.php

<?php
    session_start();
    $_SESSION['user_id'] = 1;
    require_once __DIR__.'/../vendor/dropbox-sdk/lib/Dropbox/autoload.php';
    $key = "35ihzafqgsubhdk";
    $secret = "q2ow8djsqnp72aa";
    $GLOBALS['app_name'] = "samidha/1.0";
    $GLOBALS['redirectURI'] = "https://localhost/drop/dropbox_finish.php";
    $GLOBALS['HomeURI'] = "https://localhost/drop/";

    $appInfo = new Dropbox\AppInfo($key, $secret);

    //CSRF Cross Site Request Forgery protection.
    $csrfTokenStore = new Dropbox\ArrayEntryStore($_SESSION, 'dropbox-auth-csrf-token');

    $webAuth = new Dropbox\WebAuth($appInfo, $GLOBALS['app_name'], $GLOBALS['redirectURI'], $csrfTokenStore);

    $db = new PDO('mysql:host=localhost;dbname=drop_api','root','');

    $user = $db->prepare("SELECT * FROM users WHERE id = :user_id");
    $user->execute(['user_id' => $_SESSION['user_id']]);
    $user = $user->fetchObject();
?>

dropbox_auth.php

<?php
    if ($user->token){
        $client = new Dropbox\Client($user->token, $GLOBALS['app_name'], 'UTF-8');
        $client->getAccountInfo();    
    } else {
        $authURL = $webAuth->start();
        header("Location: $authURL");
        exit();
    }
?>

dropbox_finish

<?php
    require_once "/app/start.php";
    list($accessToken) = $webAuth->finish($_GET);
    $store = $db->prepare("UPDATE users SET token = :dropbox_token"
            . "WHERE id = :user_id");
    $store->execute(['dropbox_token' => $accessToken,
            'user_id' => $_SESSION['user_id']]);
    header("Location: ".$GLOBALS['HomeURI']);
?>

Show me error in wamp server

! ) Fatal error: Uncaught exception 'Dropbox\WebAuthException_BadState' with message 'Missing CSRF token in session.' in H:\wamp\www\Drop\vendor\dropbox-sdk\lib\Dropbox\WebAuth.php on line 235
( ! ) Dropbox\WebAuthException_BadState: Missing CSRF token in session. in H:\wamp\www\Drop\vendor\dropbox-sdk\lib\Dropbox\WebAuth.php on line 235
Call Stack
#   Time    Memory  Function    Location
1   0.0010  132144  {main}( )   ...\dropbox_finish.php:0
2   0.0390  497696  Dropbox\WebAuth->finish( )

I am trying to allow my code to access the dropbox functionality with php but its getting error in dropbox_finish as shown above please help me.