致命错误：未捕获错误：Class＆＃39; Paypal \ Api \ Payment＆＃39;找不到

Question

帮帮我们！如标题中所述出现错误。一直想弄明白，但我现在还没有找到任何有用的答案。谢谢。

  

致命错误：未捕获错误：Class＆＃39; Paypal \ Api \ Payment＆＃39;在C：\ xampp \ htdocs \ paypal \ pay.php中找不到：19堆栈跟踪：在第19行的C：\ xampp \ htdocs \ paypal \ pay.php中抛出＃0 {main}

<?php

use Paypal\Api\Payment;
use Paypal\Api\PaymentExecution;

require 'app/start.php';

if(!isset($_GET['success'], $_GET['paymentId'],$_GET['PayerID'])) {
   die();
 }

if ((bool)$_GET['success'] === false){
   die();
}

$paymentId = $_GET['paymentId'];
$payerId = $_GET['PayerID'];

$payment = Payment::get($paymentId, $paypal);
$execute = new PaymentExecution();
$execute->setPayerId($payerId);

try {
    $result = $payment -> execute($execute, $paypal);
} catch (Exception $e) {
    die($e);
}
?>