Warning Undefined array key “HTTP_STRIPE_SIGNATURE”

I’m getting:

warning Undefined array key “HTTP_STRIPE_SIGNATURE”

When I visit https:/example.com/webhook.php. That is the page where I have this code:

<?php

require "..../stripe-php-11.0.0/init.php";

// Your Stripe webhook endpoint secret
$webhook_secret = "whsec_...";

// Initialize Stripe with your secret API key
StripeStripe::setApiKey("sk_test_....");

// Get the raw request body and Stripe signature header
$payload = file_get_contents("php://input");
$sig_header = $_SERVER["HTTP_STRIPE_SIGNATURE"];

try {
    // Verify the signature
    $event = StripeWebhook::constructEvent(
        $payload,
        $sig_header,
        $webhook_secret,
    );
} catch (Exception $e) {
    // Invalid signature or other error
    http_response_code(400);
    exit();
}

// Handle the event
switch ($event->type) {
    case "checkout.session.completed":
        $session = $event->data->object;

        // Process the 'checkout.session.completed' event here
        // You can retrieve information about the completed session in $session

        // Example: Send a confirmation email to the customer
        // mail($session->customer_email, 'Order Confirmation', 'Thank you for your purchase!');
        $sessionId = $session->id;

        // Implement your logic to initiate the file download here
        http_response_code(200); // Acknowledge receipt of the event
        break;

    // Add more cases to handle other types of events as needed
    // case 'invoice.paid':
    //     // Handle invoice paid event
    //     break;

    default:
        // Unknown event type
        http_response_code(200);
        break;
}

I have already configured the webhook to point to the webhook URL and I checked and a successful stripe-signature was there after a successful checkout event. It’s just not verifying it when I go to the site manually. I’ve already had events sent there I can see it on the dashboard. I obviously replaced all the '...' in my code with the correct stuff. How do I fix this?