Placing order using examples doesn't bounce browser to Klarna

I'm trying to work with the a direct integration of the Klarna Payments API. I've been following the integration guide and I'm up to the Place Order stage (https://developers.klarna.com/documentation/klarna-payments/integration-guide/place-order). I'm using their example script to place an order (https://github.com/klarna/kco_rest_php/blob/v4.x/docs/examples/PaymentsAPI/Orders/create_order.php).

Upon redirecting the user, the user is sent straight to the confirmation page rather than a Klarna page to enter their billing details (as described in the integration guide above).

I post the authorization_token to their example script using a form. Their example script responds with an array containing a redirect_url, which I redirect the script to. However, this leads to the order_confirmation page that is passed as an argument to order->create().

// The form I'm using to post to their example script:

form action="klarna_order.php" method="post"
input id="authorization_token" name="authorization_token" type="hidden"
input type="submit" value="Submit"
/form

// The part where the order is created and I redirect the page in the example // script:

try {
$order = new Klarna\Rest\Payments\Orders($connector, $authorizationToken);
$data = $order->create($data);
print_r($data);
header('Location: '.$data['redirect_url']);
die();
} catch (Exception $e) {
echo 'Caught exception: ' . $e->getMessage() . "\n";
}

The output of the klarna_order.php file (their example file) is this, which looks right:

'Array ( [order_id] => 1caa1e86-8e71-7bfa-bdd2-ff718365f1af [redirect_url] => https://klarna-payments-eu.playground.klarna.com/v1/sessions/dfa44305-aa56-7861-ac51-fee7c42ce986/redirect [fraud_status] => ACCEPTED [authorized_payment_method] => Array ( [type] => invoice ) )'

If I redirect them to the redirect_url in the response, as their guide says, the browser goes straight to the order_confirmation page instead of a Klarna page where the customer can enter their billing details.