PayPal: Getting error: "Http response code 400 when accessing https://ift.tt/2O0xHzx"

Having followed the code in a GitHub example, and some suggestions here (additional error messages, and an error method), nothing is able to get me south of: $payment->create($this->_api_context); and into the exception handling of the try block, or beyond the generic error message:

Http response code 400 when accessing https://api.sandbox.paypal.com/v1/payments/payment.

Here is the method I'm using:

public function makePayment(Request $request) {

    $payer = new Payer();

    $payer->setPaymentMethod('paypal');

    $orderForPayment = $request->get('orderForPayment');

    $orderTotal = $request->get('orderTotal');

    $orderTax = $request->get('orderTax');

    // Reference: http://udrcld.com/narrative#note-24803
    $shipping = 0.66;

    $items = [];

    foreach ($orderForPayment as $index => $item):

        $items[$index] = new Item();

        $items[$index]->setName($item['name'])
            ->setCurrency('GBP')
            ->setQuantity($item['qty'])
            ->setPrice($item['subtotal']);

    endforeach;

    $itemsList = new ItemList();

    $itemsList->setItems($items);

    $details = new Details();

    $details->setShipping($shipping)
        ->setTax($orderTax)
        ->setSubtotal($orderTotal);

    $amount = new Amount();

    $amount->setCurrency('GBP')
        ->setTotal($orderTotal + $orderTax + $shipping)
        ->setDetails($details);

    $transaction = new Transaction();

    $transaction->setAmount($amount)
        ->setItemList($itemsList)
        ->setDescription("Your transaction description.");

    $redirect_urls = new RedirectUrls();

    $redirect_urls->setReturnUrl(URL::route('getPaymentStatus'))
        ->setCancelUrl(URL::route('getPaymentStatus'));

    $payment = new Payment();

    $payment->setIntent("Sale")
        ->setPayer($payer)
        ->setRedirectUrls($redirect_urls)
        ->setTransactions(array($transaction));

    //dd($payment->create($this->_api_context)); exit;

    try {

        $payment->create($this->_api_context);

    } catch (PayPal\Exception\PayPalConnectionException $ex) {

        // Prints the Error Code
        echo $ex->getCode();

        // Prints the detailed error message
        echo $ex->getData();

        echo $this->PayPalError($ex);

    } catch (Exception $ex) {

        echo $this->PayPalError($ex);

    }

    foreach ($payment->getLinks() as $link) {

        if ($link->getRel() == 'approval_url') {

            $redirect_url = $link->getHref();

            break;

        }

    }

    // Add payment ID to the session.
    \Session::put('paypal_payment_id', $payment->getId());
    if (isset($redirect_url)) {

        // Redirect to PayPal.
        return Redirect::away($redirect_url);

    }

    \Session::put('error', "Unknown error occurred");

    return Redirect::route('makePayment');

}

I've checked the values down through the method, to make sure they're integers and not strings. I've tried swapping the currencies around between GBP and USD.

I've not used PayPal's API before, so it's possible I've got something wrong, but the generic error is killing the debug process.

via Chebli Mohamed