WHMCS custom function - Stripe payment gateway error

I am trying to add a custom function in WHMCS, by creating a file addstripecard.php, the idea is based on a given customer token (something like cus_1GPkTFJEegxX3y3c123456) and then try to attach it to a card. The custom function is accessed from an API.

First, I'm trying to access the Stripe library, by using these commands :

$gateway = new \WHMCS\Module\Gateway();
$gateway->load("stripe");

PS: the codes were taken from https://github.com/WHMCSCare/WHMCS-7.8.0-decoded/blob/e7446479de49a28c8801d4c0c95f4cae22dcff33/modules/gateways/stripe/lib/StripeController.php

But, after running the above commands, I got this error message :

FastCGI sent in stderr: "PHP message: [WHMCS Application] ERROR: TypeError: Argument 2 passed to WHMCS\Api\ApplicationSupport\Http\ResponseFactory::factory() must be of the type array, string given,

called in /www/xxxxxx.xxx/billing/vendor/whmcs/whmcs-foundation/lib/Api/ApplicationSupport/Route/Middleware/HandleProcessor.php on line 0 

and defined in /www/xxxxxx.xxx/billing/vendor/whmcs/whmcs-foundation/lib/Api/ApplicationSupport/Http/ResponseFactory.php:0

Stack trace: #0 /www/xxxxxx.xxx/billing/vendor/whmcs/whmcs-foundation/lib/Api/ApplicationSupport/Route/Middleware/HandleProcessor.php(0): WHMCS\Api\ApplicationSupport\Http\ResponseFactory::factory(Object(WHMCS\Api\ApplicationSupport\Http\ServerRequest), '{"legacyGateway...') #1 /www/xxxxxx.xxx/billing/vendor/whmcs/whmcs-foundation/lib/Route/Middleware/Strategy/DelegatingMiddlewareTrait.php(0): WHMCS\Api\ApplicationSupport\Route\Middleware\HandleProcessor->_process(Object(WHMCS\Api\ApplicationSupport\Http\ServerRequest), Object(Middlewares\Utils\Delegate)) #2 /www/dat" while reading response header from upstream, client: 172.81.129.13, 

server: xxxxxx.xxx.net, 

request: "POST /billing/includes/api.php HTTP/1.1",

upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:",

host: "xxxxxx.xxx"

Full codes:

<?php

if (!defined("WHMCS")) {
    die("This file cannot be access directly!");
}

function addStripeCard()
{
    // card parameters
    $cardNumber       = $_POST['cardnumber'];
    $cardExpiryDate   = $_POST['cardexpirydate'];
    $remoteToken      = $_POST['remotetoken'];
    $billingContactId = $_POST['billingcontactid'];
    $description      = $_POST['description'];
    $cardType         = $_POST['cardtype'];
    $cardStartDate    = $_POST['cardstartdate'];
    $cardIssueNumber  = $_POST['cardissuenumber'];
    $clientId         = $_POST['clientid'];

    try {
        $gateway = new \WHMCS\Module\Gateway();

        $gateway->load("stripe");

        return json_encode($gateway);
    } catch (Exception $e) {
        $error = [
            'result'  => 'error',
            'message' => $e->getMessage(),
            'file'    => $e->getFile(),
            'line'    => $e->getLine(),
        ];

        return $error;
    }
}

try {
    $data = addStripeCard();

    $apiresults = $data;

} catch (Exception $e) {
    $error = [
        'result'  => 'error',
        'message' => $e->getMessage(),
        'file'    => $e->getFile(),
        'line'    => $e->getLine(),
    ];

    $apiresults = $error;
}

Does anyone have a clue on what's going on? I've tried googling many times, but couldn't find any answers.

I have found the answer.

When returning response to the client we should at least follow this format :

$apiresults = array('result' => $result, 'message' => $message);

it can be extended to :

$apiresults = array('result' => $result, 'message' => $message, 'data' => $data);

so I have modified the above codes to :

<?php

if (!defined("WHMCS")) {
    die("This file cannot be access directly!");
}

function addStripeCard()
{
    // card parameters
    $cardNumber       = $_POST['cardnumber'];
    $cardExpiryDate   = $_POST['cardexpirydate'];
    $remoteToken      = $_POST['remotetoken'];
    $billingContactId = $_POST['billingcontactid'];
    $description      = $_POST['description'];
    $cardType         = $_POST['cardtype'];
    $cardStartDate    = $_POST['cardstartdate'];
    $cardIssueNumber  = $_POST['cardissuenumber'];
    $clientId         = $_POST['clientid'];

    try {
        $gateway = new \WHMCS\Module\Gateway();

        $gateway->load("stripe");

        return json_encode($gateway);
    } catch (Exception $e) {
        $error = [
            'result'  => 'error',
            'message' => $e->getMessage(),
            'file'    => $e->getFile(),
            'line'    => $e->getLine(),
        ];

        return $error;
    }
}

try {
    $data = addStripeCard();

    $apiresults = array('result' => 'success', 'message' => 'success message', 'data' => $data);
} catch (Exception $e) {
    $error = [
        'result'  => 'error',
        'message' => $e->getMessage(),
        'file'    => $e->getFile(),
        'line'    => $e->getLine(),
    ];

    $apiresults = $error;
}

after that the error gone, the API endpoint which access the custom function receives 200 response.