تست درگاه زرین پال در php

سلام دوستان من تو یه پروژه تستی php میخام تست درگاه بگیرم با زرین پال
کدهامو میزارم و همینطور خطا لطفا راهنمایی کنید خیلی ضررویه
ممنونم
صفحه payment.php

<?php

$data = array("merchant_id" => "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
    "amount" => 1000,
    "callback_url" => "http://localhost/verify.php",
    "description" => "خرید تست",
    "metadata" => [ "email" => "[email protected]","mobile"=>"09121234567"],
    );
$jsonData = json_encode($data);
$ch = curl_init('https://sandbox.zarinpal.com/pg/v4/payment/request.json');
curl_setopt($ch, CURLOPT_USERAGENT, 'ZarinPal Rest Api v1');
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($ch, CURLOPT_POSTFIELDS, $jsonData);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Content-Length: ' . strlen($jsonData)
));

$result = curl_exec($ch);
$err = curl_error($ch);
$result = json_decode($result, true, JSON_PRETTY_PRINT);
curl_close($ch);

if ($err) {
    echo "cURL Error #:" . $err;
} else {
    if (empty($result['errors'])) {
        if ($result['data']['code'] == 100) {
            header('Location: https://sandbox.zarinpal.com/pg/StartPay/' . $result['data']["authority"]);
        }
    } else {
         echo'Error Code: ' . $result['errors']['code'];
         echo'message: ' .  $result['errors']['message'];

    }
}
?>

verify.php

<?php
$Authority = $_GET['Authority'];
$data = array("merchant_id" => "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx", "authority" => $Authority, "amount" => 1000);
$jsonData = json_encode($data);
$ch = curl_init('https://sandbox.zarinpal.com/pg/v4/payment/verify.json');
curl_setopt($ch, CURLOPT_USERAGENT, 'ZarinPal Rest Api v4');
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($ch, CURLOPT_POSTFIELDS, $jsonData);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Content-Length: ' . strlen($jsonData)
));

$result = curl_exec($ch);
curl_close($ch);
$result = json_decode($result, true);
if ($err) {
    echo "cURL Error #:" . $err;
} else {
    if ($result['data']['code'] == 100) {
        echo 'Transation success. RefID:' . $result['data']['ref_id'];
    } else {
        echo'code: ' . $result['errors']['code'];
        echo'message: ' .  $result['errors']['message'];
    }
}
?>

وقتی روی پرداخت میزنم خطای

Warning: Trying to access array offset on value of type null in C:\xampp\htdocs\shoptest\payment.php on line 31

Warning: Trying to access array offset on value of type null in C:\xampp\htdocs\shoptest\payment.php on line 31

دوستان لطفا راهنمایی کنید مشکل کجاست و چیکار باید کرد؟