public function defaultCallback($handler)
    {
        $exception = $handler->getLastError();
        if (php_sapi_name() != "cli") {
            $message   = htmlspecialchars($exception->getMessage());
            $trace     = nl2br(htmlspecialchars($exception->getTraceAsString()));
            echo "<!DOCTYPE html><html><head><title>Error</title></head><body>",
                "<h1>{$message}</h1><p>{$trace}</p></body><html>";
        } else {
            echo $exception->getMessage() . PHP_EOL,
                "-----------" . PHP_EOL,
                $exception->getTraceAsString() . PHP_EOL;
        }
    }