Passing id checkbox to modal window

i wanted to pass the id from the checkbox to modal once i pressed "bonus" button. I wanted to insert bonus value to only the one who were picked inside the modal window. I just don't know if my coding is correct.

For Php Insert to DB `if(isset($_REQUEST['bonus'])) {

    //$ch = "check";
    $bonus = $_POST['bonus'];
    $chk= $_REQUEST['check_list'];
    $a=implode(",",$chk);

    echo $a;
    $query2 = "Update payroll set payroll_bonus = ('$bonus') where user_id in ($a) ";

    $result2 = mysqli_query($conn, $query2);

}`

For Bonus Button

<!-- Button Bonus -->
        <div class="row mb-2">
        <div class="col-sm-12" align='right'>
                <a href="#ModalCenter" data-toggle="modal" data-id="<?php echo $row['user_id']?>">
                <button type="submit" name= "hantar" class="btn btn-sm btn-primary">Bonus</button>
                </a>
        </div>
        </div>
        <!-- /.Button Bonus -->

For Modal Bonus

<!-- Modal Bonus -->

Bonus Pekerja ×

            <div class="card-body">
            <div class="form-group">
                <label>Masukkan Bonus (%)</label>
                <input name="bonus" type="text" class="form-control" >
                <input type="hidden" name="bonus">
            </div>
            </div>

            <!-- /.card-body -->

        <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <button class="btn btn-primary" name="bonus">Save changes</button>
        </div>
        </div>
        </div>
        </div>
        </div>

For Bonus Javascript

<script>

$('#ModalCenter').on('show.bs.modal', function(e) { var bonus = $(e.relatedTarget).data('bonus');

$(e.currentTarget).find('input[name="bonus"]').val(bonus);
  //$('#message').html(id);
});

Bonus Page Pic