Has anyone successfully managed to configure a PHP class method to retrieve a CheckoutId for a Sumup Payment Widget

Question

Our HTML/Javascript client sends a request to our PHP proxy Class and method to invoke the communication with the Sumup API.

The Sumup Developer Documentation appears to require three steps:

1. Use Curl to obtain an Authorisation Code from the API
2. Use that Code with the PHP SDK for the first time to retrieve an Access Token
3. Then use that Token with the PHP SDK for the second time to retrieve a Checkout Id.

Finally the Checkout ID is returned to the HTML/Javascript client to be used by the Payment Widget

I have spent many days trying to interpret the Sumup Developer documentation. There are no examples provided and support@sumup is prompt but only interested in answering detailed questions.

This is the first time I have needed to use Stackoverflow for a question for at least 10 years. I hope a colleague has achieved success and can provide some assistance.

    $checkoutId = ""; $token = ""; $code = "";

    $curl = new Curl\Curl();
    $curl->get('https://api.sumup.com/authorize', [
        'response_type' => 'code',
        // 'client_id' => 'cc_classic_*************',
        'client_id' => 'sup_sk_*********************',
        'redirect_uri' => 'https://api.*****',
    ]);

    if($curl->isSuccess()) {
        // do something with response
        $code = $curl->response;
    } else {
        $code = 'Curl error';
    }
    // ensure to close the curl connection
    $curl->close();

    $sumup = new \SumUp\SumUp([
        'app_id'     => "cc_classic_***********", // required,
        'app_secret' => "cc_sk_classic_*******************",
        'grant_type' => 'authorization_code', 
        'code' => $code,
    ]);
    $accessToken = $sumup->getAccessToken();
    $token = $accessToken->getValue();

    $sumupb = new \SumUp\SumUp([
        'app_id'     => "cc_classic_*****************", // required,
        'app_secret' => "cc_sk_classic_********************",
        'code'       => $token,
    ]);
    $checkoutService = $sumupb->getCheckoutService();
    $checkoutResponse = $checkoutService->create($this->rq->data['amount'], $this->rq->data['currency'], $this->rq->data['id'], $this->mdl['email']);
    $checkoutId = $checkoutResponse->getBody()->id;
    //  pass the $chekoutId to the front-end to be processed