DropboxWebhook.php

<?php

class DropboxWebhook
{
   /**
     * Handle the dropbox webhook request
     */
    public function handle()
    {
        global $container;

        header('Content-Type: text/plain');

        // Check get parameter
        // Necessary for enabling the webhook via dropbox' app console
        if (($challenge = \Input::get('challenge'))) {
            die($challenge);
        }

        $rawData   = file_get_contents('php://input');
        $json      = json_decode($rawData);
        $appSecret = $container['dropbox.appSecret'];

        // Check the signature for a valid request
        if ($_SERVER['HTTP_X_DROPBOX_SIGNATURE'] !== hash_hmac('sha256', $rawData, $appSecret)) {
            header('HTTP/1.0 403 Forbidden');
            die('Invalid request');
        }

        // Return a response to the client before processing
        // Dropbox wants a response quickly
        header('Connection: close');
        ob_start();
        header('HTTP/1.0 200 OK');
        ob_end_flush();
        flush();

        // Do all the stuff you want to
    }
}

handle-dropbox-webhook.php

<?php

// Run the controller
$controller = new DropboxWebhook();
$controller->handle();