Check if Stripe Payment Failed or Succeeded

Question

In my iOS app, I'm using the following code to make a connection to a Stripe PHP web page of mine in order to process a customer's transaction:

   NSString *dataUrl = [NSString stringWithFormat:@"http://www.somesite?stripeToken=%@",token.tokenId];
    NSURL *url = [NSURL URLWithString:dataUrl];

    // 2
    NSURLSessionDataTask *downloadTask = [[NSURLSession sharedSession]
                                          dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
}];

    // 3
    [downloadTask resume];

The code I've used to receive the above response is the following:

<?php

require_once('vendor/autoload.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("private");

// Get the credit card details submitted by the form
$token = $_POST['stripeToken'];

// Create the charge on Stripe's servers - this will charge the user's card
try {

    $charge = \Stripe\Charge::create(array(
  "amount" => 1000, // amount in cents, again
  "currency" => "cad",
  "source" => $token,
  "description" => "Example charge")
);
    $string = "success";
    echo json_encode($string); 

} catch(\Stripe\Error\Card $e) {
    $string = "The card has been declined.";
    echo json_encode($string); 
}

?>

What I can't seem to do is check if the payment went through properly or not. I've currently got the method below and I just want to see if I'm getting ANY response from the server at all. But the NSLog is not printing anything, so I don't think the method is being called. When I check my Stripe Log, it accurately tells me when payments failed/went through (so that part works fine). I simply am not able to get any response back from my PHP page for my iOS app.

-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    NSLog(@"RESPONSE: %@",data);
}

How can I fix that?