在表单提交时从mysql中获取post复选框值

我想制作一个简单的注册系统。用户选择要注册的事件，设置要注册的人数，然后单击提交。接下来，他被重定向到注册页面，系统在该页面生成注册表单并修改查询，以便将记录添加到事件专用的表中。

我在提交复选框值时遇到问题，我一直收到未定义索引错误

请咨询

这是代码：（HTML）

<form action="register-form.php" method="post">
        <table class="table table-bordered table-hover">
            <tbody>
                <?php show_active_events(); ?>
            </tbody>
        </table>
        <div class="form-group col-md-12" id="counter">
            <label for="select">Wybierz ilość osób do rejestracji</label>
            <select class="form-control" name="person_number">
            <option>1</option>
            <option>2</option>
            <option>3</option>
            <option>4</option>
            <option>5</option>
            <option>6</option>
            <option>7</option>
            <option>8</option>
            <option>9</option>
            </select>
        </div>
        <button type="submit" class="btn btn-lg btn-primary col-md-12 pull-right" id="event-table-btn" name="submit">Dalej <span class="glyphicon glyphicon-chevron-right" aria-hidden="true"></span></button>
    </div>
    </form>

SHOW_ACTIVE_EVENTS FUNCTION

function show_active_events() {
    global $conn;
    $query = "SELECT * FROM events WHERE status NOT LIKE '0' ";
    $showEvents = mysqli_query($conn, $query);
    while($row = mysqli_fetch_assoc($showEvents)) { ?>
        <tr>
            <td width="90%">
<?php
        echo $row['name'];
        echo "<br />";
        echo $row['date'];
        echo "<br />";
        echo  $row['time'];
        echo "<br />";
?>
            <div class="accordion">Szczegóły ...
                <div class="accordion-content hide">
                    <p><?php echo $row['comment']; ?></p>
                </div>    
            </div>
            </td>
            <td width="10%">
            <div class="ck-button">
                    <label>
                        <input type="checkbox" class="ck" name="event" id="event" value="<?php echo $row['name'];?>"><span>Wybierz</span>
                    </label>
            </div>
            </td>
        </tr>
<?php }}?>

注册格式。PHP

<?php
if (isset($_POST['submit'])) {
$persons = $_POST['person_number'];
$event = $_POST['event'];
echo $event;
echo $persons;
}
?>