add_action('init', function () {
  if (isset($_GET['action']) && $_GET['action'] === 'fsp_callback') {
    $file = WP_CONTENT_DIR . '/uploads/fsp-callback.log';
    $data = [
      'time'   => gmdate('c'),
      'ua'     => $_SERVER['HTTP_USER_AGENT'] ?? '',
      'qs'     => $_SERVER['QUERY_STRING'] ?? '',
      // don't log body; just keys for safety
      'post'   => array_keys($_POST ?: []),
    ];
    file_put_contents($file, print_r($data, true) . "\n---\n", FILE_APPEND);
  }
});