类未找到错误但类可用

I have my folder structure like

1) samples folder contains all the requested file

2) lib folder contains all the required classes

When i am running a particular file RequestPermissionsReceipt.php file it is showing error that

Fatal error: Class 'PayPal\Types\Common\RequestEnvelope' not found in samples/RequestPermissionsReceipt.php on line 59

but this class is available in the lib\PayPal\Types\Common folder.

What should i do to remove this error ? Code of my RequestPermissionsReceipt file is

use PayPal\Service\PermissionsService;
use PayPal\Types\Common\RequestEnvelope;
use PayPal\Types\Perm\RequestPermissionsRequest;

require_once('PPBootStrap.php');

$serverName = $_SERVER['SERVER_NAME'];
$serverPort = $_SERVER['SERVER_PORT'];
$url =  dirname('http://'.$serverName.':'.$serverPort.$_SERVER['REQUEST_URI']);
$returnURL = $url."/GetAccessToken.php";
$cancelURL = $url. "/RequestPermissions.php";

$scope = array();
   if(isset($_POST['chkScope'])) {
      $i = 0;
      foreach ($_POST['chkScope'] as $value) {
          $scope[$i++] = $value;
      }
   }
$requestEnvelope = new RequestEnvelope("en_US");