if (isset($_POST['submitBtn'])) {
        $messageText = $_POST['messageText'];
        $receiverName = $_POST['receiverName'];

        if (!empty($messageText)) {
            $sql = "SELECT `id` FROM `users` WHERE user_name = '$receiverName'";
            $result = $conn->query($sql);
            $user = $result->fetch();
            $userId = $user['id'];
            $senderId = $_SESSION['user_id'];
            $sqlToInsertMessage = "INSERT INTO `messages`(`id`, `text`, `receiver_user_id`, `sender_user_id`) VALUES (null, '$messageText', '$userId', '$senderId')";
            $messageInstertingResult = $conn->query($sqlToInsertMessage);
        }

    }