Very simple PHP webhook for Mailgun that emails you on errors.

mailgun-webhook.php

<?php
$key = "<API Key>";
$from = "info@example.com";
$to = "dave@example.com";


if ($_SERVER['REQUEST_METHOD'] == 'POST') {
	if(isset($_POST['timestamp']) && isset($_POST['token']) && isset($_POST['signature']) && hash_hmac('sha256', $_POST['timestamp'] . $_POST['token'], $key) === $_POST['signature'])
	{
		if($_POST['event'] == 'complained') {
			$subject = "[Mailgun] Spam Complaint";
			$body = "Recipient: " . $_POST['recipient'] . "\nDomain: " . $_POST['domain'] . "\n\nMessage-headers: " . $_POST['message-headers'] . "\n";
			mail($to, $subject, $body, "From: " . $from,"-f". $from);
		}elseif($_POST['event'] == 'bounced'){
			$subject = "[Mailgun] Bounced Email";
			$body = "Recipient: " . $_POST['recipient'] . "\nDomain: " . $_POST['domain'] . "\nCode: " . $_POST['code'] . "\nError: " . $_POST['error'] . "\nNotification: " . $_POST['notification'] . "\n\nMessage-headers: " . $_POST['message-headers'] . "\n";
			mail($to, $subject, $body, "From: " . $from,"-f". $from);
		}elseif($_POST['event'] == 'dropped'){
			$subject = "[Mailgun] Failed Email";
			$body = "Recipient: " . $_POST['recipient'] . "\nDomain: " . $_POST['domain'] . "\nCode: " . $_POST['code'] . "\nReason: " . $_POST['reason'] . "\nDescription: " . $_POST['description'] . "\n\nMessage-headers: " . $_POST['message-headers'] . "\n";
			mail($to, $subject, $body, "From: " . $from,"-f". $from);
		}
	}
}
header('X-PHP-Response-Code: 200', true, 200);
?>

[In case anyone is similarly confused]

How do you read other data in your example like the message ID. I've tried various options and do not get a result.

Eg:
$msg_id = $event->event_data->message->headers->{'message-id'};

$data->message->headers->{'message-id'}