Setting PayPal PHP Application live

I followed some dudes tutorial on how to Charge People with PayPal using the PHP SDK. It all works like a charm as long as I'm in Testmode, but if I cange my API Keys to the "live" ones, I only get an HTTP Error 401. I understand that this is because I have to set the Application to "Live".

I followed the Guide on PayPals GitHub Page. (https://github.com/paypal/PayPal-PHP-SDK/wiki/Going-Live)

Dont worry, I dont do this for the acutal Web, I just want to get back into PHP and it drives me nuts that I am not able to configure this script.

I have the following "charge.php":

    require 'app/start.php';
use PayPal\Api\Payer;
use PayPal\Api\Item;
use PayPal\Api\ItemList;
use PayPal\Api\Details;
use PayPal\Api\Amount;
use PayPal\Api\Transaction;
use PayPal\Api\RedirectUrls;
use PayPal\Api\Payment;

if(!isset($_POST['product'])){
    echo $_POST['product'];
    die("Nope");
}

    $product="Premium";
    $price="1.00";
    $shipping="0.00";
    $total = $price+$shipping;

    $payer = new Payer();
    $payer->setPaymentMethod('paypal');

    $item = new Item();
    $item->setName($product)
    ->setCurrency('EUR')
    ->setQuantity(1)
    ->setPrice($price);

    $itemList = new ItemList();
    $itemList->setItems([$item]);

    $details = new Details();
    $details->setShipping($shipping)
    ->setSubtotal($price);

    $amount = new Amount();
    $amount->setCurrency('EUR')
    ->setTotal($total)
    ->setDetails($details);

    $transaction = new Transaction();
    $transaction->setAmount($amount)
    ->setItemList($itemList)
    ->setDescription($product)
    //UserID 
    ->setInvoiceNumber(rand());

    $redirectUrls = new RedirectUrls();
    $redirectUrls->setReturnUrl('/paid.php?sucess=true')
    ->setCancelUrl('/paid.php?sucess=false');

    $payment = new Payment();
    $payment->setIntent('sale')
    ->setPayer($payer)
    ->setRedirectUrls($redirectUrls)
    ->setTransactions([$transaction]);

    try{
        $payment->create($paypal);

    } catch (Exception $e){
        die($e);
    }

    $approvalUrl=$payment->getApprovalLink();
    header("Location: {$approvalUrl}");

And the following "start.php" with my API Credentials in it:

require 'vendor/autoload.php';

define('SITE_URL', '/charge.php');

$paypal = new \PayPal\Rest\ApiContext(
    new \PayPal\Auth\OAuthTokenCredential(
    'xx',
    'xx'
    )
);

Can someone give me a hint, where I need to confige the App to use PayPals Live API? I tried it in the carge.php, but I only get the PHP Error, that I use an undefined variable.

Im talking about the following Snipit from PayPals Github:

$apiContext->setConfig(
      array(
        ...
        'mode' => 'live',
        ...
      )
);

Any help would be much appreciated. Cheers, Flo