Sends email when a Stripe subscription payment fails

invoice.payment_failed.php

<?php

if (!isset($_POST)) {
    die("Request cannot be empty.");
}

$input = @file_get_contents("php://input");
$event_json = json_decode($input, 1);

$data = $event_json["data"]["object"];

if ($event_json['type'] == "invoice.payment_failed") {

    $amount_due  = strtoupper($data["currency"]) . " " . $data["amount_due"];
    $customer_id = $data["customer"];

    $subject = "Subscription payment failed";
    $to = "email@domain.com"; // replace with your email;
    $message = "<p style='font-size: 1em'> Subscription payment ($amount_due) for customer $customer_id has failed.<p>";
    
    mail($to, $subject, $message, "Content-Type: text/html; charset=\"UTF-8\"\r\n");
}
?>