function send_notification($pdo, $type, $title, $message, $link = null, $to_user = null, $to_vendor = null, $to_staff = null) {
    
    $stmt = $pdo->prepare("
        INSERT INTO notifications (user_id, vendor_id, staff_id, type, title, message, link, is_read)
        VALUES (?, ?, ?, ?, ?, ?, ?, 0)
    ");

    $stmt->execute([
        $to_user,
        $to_vendor,
        $to_staff,
        $type,
        $title,
        $message,
        $link
    ]);
}