Unable to fetch data (Client Email, ID, Phone Number, Address etc.)

[sysconfig]

Hi,

i was trying to create a payment gateway that i will host on another website, there i created a php file but that php file was unable to fetch data from my whmcs.
Here is the related codes:
 

$whmcs_url = 'https://my.domain.com/includes/api.php';
$whmcs_username = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX';
$whmcs_password = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX';

// Function to call WHMCS API
function whmcs_api($action, $data) {
    $postFields = array();
    $postFields['username'] = $GLOBALS['whmcs_username'];
    $postFields['password'] = md5($GLOBALS['whmcs_password']);
    $postFields['action'] = $action;
    $postFields['responsetype'] = 'json';
    $postFields += $data;

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $GLOBALS['whmcs_url']);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_TIMEOUT, 30);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($postFields));
    $response = curl_exec($ch);
    curl_close($ch);

    return json_decode($response, true);
}

$clientEmail = $customerData['email']; // Replace with the customer's email
$clients = whmcs_api('GetClients', array('search' => $clientEmail));

if (isset($clients['numreturned']) && $clients['numreturned'] > 0) {
    $customerId = $clients['clients']['client'][0]['id'];
    $customerData = $clients['clients']['client'][0];
} else {
    echo "Customer not found in WHMCS.";
    exit;
}

$invoices = whmcs_api('GetInvoices', array('userid' => $customerId, 'status' => 'Unpaid'));

if (isset($invoices['numreturned']) && $invoices['numreturned'] > 0) {
    $invoiceId = $invoices['invoices']['invoice'][0]['id'];
    $invoiceAmount = $invoices['invoices']['invoice'][0]['total'];
    echo "Redirecting...<br>";
} else {
    echo "No unpaid invoice found for the customer.";
    exit;
}

$store_id = 'XXXXXXXXX';
$store_passwd = 'XXXXXXXXXXX';
$submit_url = 'https://example.com/gwprocess/v4/api.php';
$easy_url = 'https://example.com/embed.min.js';
$success_url = 'https://example.com/success.php';
$fail_url = 'https://example.com/fail.php';
$cancel_url = 'https://example.com/cancel.php';
$ipn_url = 'https://example.com/ipn.php';

// Prepare data for payment request
$post_data = array();
$post_data['store_id'] = $store_id;
$post_data['store_passwd'] = $store_passwd;
$post_data['total_amount'] = $invoiceAmount;
$post_data['currency'] = 'USD'; // Set your desired currency
$post_data['tran_id'] = $invoiceId;
$post_data['success_url'] = $success_url;
$post_data['fail_url'] = $fail_url;
$post_data['cancel_url'] = $cancel_url;
$post_data['ipn_url'] = $ipn_url;
$post_data['cus_name'] = $customerData['firstname'] . ' ' . $customerData['lastname'];
$post_data['cus_email'] = $customerData['email'];
$post_data['cus_phone'] = $customerData['phonenumber'];
$post_data['cus_add1'] = $customerData['address1'];
$post_data['cus_city'] = $customerData['city'];
$post_data['cus_state'] = $customerData['state'];
$post_data['cus_postcode'] = $customerData['postcode'];
$post_data['cus_country'] = $customerData['country'];
$post_data['value_a'] = 'Payment for Invoice ' . $invoiceId;
$post_data['value_b'] = 'returnurl'; // Replace with your desired return URL
$post_data['shipping_method'] = 'NO';
$post_data['num_of_item'] = '1';
$post_data['product_name'] = 'Invoice ' . $invoiceId;
$post_data['product_profile'] = 'general';
$post_data['product_category'] = 'Domain-Hosting';

if (isset($customerData['firstname']) && isset($customerData['lastname'])) {
    $post_data['cus_name'] = $customerData['firstname'] . ' ' . $customerData['lastname'];
}
if (isset($customerData['email'])) {
    $post_data['cus_email'] = $customerData['email'];
}
if (isset($customerData['phonenumber'])) {
    $post_data['cus_phone'] = $customerData['phonenumber'];
}
if (isset($customerData['address1'])) {
    $post_data['cus_add1'] = $customerData['address1'];
}
if (isset($customerData['city'])) {
    $post_data['cus_city'] = $customerData['city'];
}
if (isset($customerData['state'])) {
    $post_data['cus_state'] = $customerData['state'];
}
if (isset($customerData['postcode'])) {
    $post_data['cus_postcode'] = $customerData['postcode'];
}
if (isset($customerData['country'])) {
    $post_data['cus_country'] = $customerData['country'];
}

$hidden_fields = '';
foreach ($post_data as $key => $value) {
    $hidden_fields .= '<input type="hidden" name="' . $key . '" value="' . htmlentities($value) . '">';
}

 

Getting errors:
 

Notice: Undefined variable: customerData in /public_html/pay/pay.php on line 47
\
Notice: Undefined index: phonenumber in /public_html/pay/pay.php on line 93
Notice: Undefined index: address] in /public_htm/pay/pay.php on line 94
Notice: Undefined index: city in /public_html/pay/pay.php on line 95

Notice: Undefined index: state in /public_html/pay/pay.php on line 96

Notice: Undefined index: postcode in /public_html/pay/pay.php on line 97

Notice: Undefined index: country in /public_html/pay/pay.php on line 98