Obj-C / PHP - 值不能正确发布？服务器返回状态码500

我正在尝试使用以下代码将交易数据从我的iOS应用程序发布到我的php后端（我发布条带令牌） . 但是，当我这样做时，服务器返回状态代码500错误 . 知道为什么这是/我在下面做错了什么？

payment.php (backend)

<?php


// Set your secret key: remember to change this to your live secret key in production
// See your keys here: https://dashboard.stripe.com/account/apikeys
\Stripe\Stripe::setApiKey("secretkeyhere");

// Token is created using Checkout or Elements!
// Get the payment token ID submitted by the form:
$token = $_POST['stripeToken'];
$charge = \Stripe\Charge::create([
    'amount' => 999,
    'currency' => 'cad',
    'description' => 'Example charge',
    'source' => $token,
]);


?>

ViewController.m

NSString *curr = @"CAD";
              NSURL *url = [NSURL URLWithString:@"http://url.com/payment.php"];
                NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
                request.HTTPMethod = @"POST";
                NSString *body     = [NSString stringWithFormat:@"stripeToken=%@", token.tokenId];

                request.HTTPBody   = [body dataUsingEncoding:NSUTF8StringEncoding];


                NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
                NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];
                NSURLSessionDataTask *task =
                [session dataTaskWithRequest:request
                           completionHandler:^(NSData *data,
                                               NSURLResponse *response,
                                               NSError *error) {
                               if (error)
                               {
                                        completion(PKPaymentAuthorizationStatusFailure);
                                   ;
                               }
                               else
                               {
                                   NSLog(@"THE RESPONSE IS %@", response);
                                      completion(PKPaymentAuthorizationStatusSuccess);
                                     NSLog(@"Server success!");
                                     NSLog(@"Stripe success!");
                               }
                           }];
                [task resume];