Trying to insert multiple checkbox values in separate rows in MySQL table in PHP not working [duplicate]

I have a simple form in PHP like below:

  <div class="col-md-4 ip"><label>Select Class</label><br>
            <?php
$ret=mysqli_query($con,"select * from class");
$cnt=1;
while ($row=mysqli_fetch_array($ret)) {

?>

<div class="form-check">
  <input name="classname[]" class="form-check-input" type="checkbox" value="<?php echo $row['class_name'];?>" id="flexCheckDefault">
  <label class="form-check-label" for="flexCheckDefault">
    <?php echo $row['class_name'];?>
  </label>
</div>

    <?php }?>
        </div>

Now I want the user selected checkboxes to get inserted in database in separate rows, so I did the following PHP code:

$classname=$_POST['classname'];

foreach ($classname as $pop ){
$ins_query="insert into study_material (`classname`) values ('$pop')";
}

Now here the issue is only the last selected checkbox value is getting inserted and others are not.