PHP - 更新sql数据

Question

i'm new to php and i have this php script, but for some reason its not working. What i want is when the checkbox is checked, the value of the specific table will increase by 1 after i click the 'submit' button.

php:

<?php
if(isset($_POST['submit'])){//to run PHP script on submit
if(!empty($_POST['check_list'])){
foreach($_POST['check_list'] as $selected){
$sql = "UPDATE users SET marks = marks + 2 WHERE id = $selected";
mysqli_query($sql);
}
}
}
?>

and here is my table code which collect the data:

<form action="#" method="post">
    <table class='default'>
    <tr>
        <th>Delete</th><th>Username</th><th>Email</th><th>First Name</th><th>Last Name</th><th>Join Date</th><th>Last Sign In</th><th>Logins</th>
    </tr>
        <tbody>

<?php foreach ($userData as $v1) { ?>

            <tr>
            <td><input type="checkbox" name="check_list[]" value="<?=$v1->id?>" /></td>
            <td><?=$v1->username?></td>
            <td><?=$v1->fname?></td>
            <td><?=$v1->lname?></td>
            <td><?=$v1->marks?></td>
            <td><?=$v1->join_date?></td>
            </tr>
            <?php } ?>

        </tbody>
    </table>
    <input type="submit" name="submit" value="Submit"/>
</form>

where am i doing wrong?