PHP issue

I need some help with this. I am trying to update the column to 'complete' when the check box is checked. What is wrong in my code?


action.php:

<?php
session_start();
include_once('dbConfig.php');
if(isset($_POST['bulk_UPDATE_submit'])){
    $idArr = $_POST['checked_id'];
    foreach($idArr as $id){
        mysqli_query($conn,"UPDATE learning_scorm_tracking SET lesson_status = 'incomplete' WHERE idscorm_tracking=".$id);
    }
    $_SESSION['success_msg'] = 'Users have been UPDATED successfully.';
    header("Location:index.php");
}

?>

index.php:

<!DOCTYPE html>
<html>
<head>
<title>Delete multiple rows in PHP by CodexWorld</title>
<link rel="stylesheet" href="style.css"/>
<script type="text/javascript" src="jquery.min.js"></script>
<script type="text/javascript">
function update_confirm(){
    var result = confirm("Are you sure to update student?");
    if(result){
        return true;
    }else{
        return false;
    }
}

$(document).ready(function(){
    $('#select_all').on('click',function(){
        if(this.checked){
            $('.checkbox').each(function(){
                this.checked = true;
            });
        }else{
             $('.checkbox').each(function(){
                this.checked = false;
            });
        }
    });

    $('.checkbox').on('click',function(){
        if($('.checkbox:checked').length == $('.checkbox').length){
            $('#select_all').prop('checked',true);
        }else{
            $('#select_all').prop('checked',false);
        }
    });
});
</script>
</head>

<body>

<?php session_start(); if(!empty($_SESSION['success_msg'])){ ?>
<div class="alert alert-success"><?php echo $_SESSION['success_msg']; ?></div>
<?php unset($_SESSION['success_msg']); } ?>
<?php
include_once('dbConfig.php');
$query = mysqli_query($conn,"SELECT * FROM learning_scorm_tracking WHERE lesson_status = 'completed'");
?>
<form name="bulk_action_form" action="action.php" method="post" onSubmit="return update_confirm();"/>
    <table class="bordered">
        <thead>
        <tr>
            <th><input type="checkbox" name="select_all" id="select_all" value=""/></th>
            <th>Student ID</th>
            <th>User Name</th>
            <th>Status</th>
            <th>Credit</th>
            <th>Time spent on course</th>
        </tr>
        </thead>
        <?php
            if(mysqli_num_rows($query) > 0){
                while($row = mysqli_fetch_assoc($query)){
        ?>
        <tr>
            <td align="center"><input type="checkbox" name="checked_id[]" class="checkbox" value="<?php echo $row['idscorm_tracking']; ?>"/></td>
            <td><?php echo $row ['idUser'];?></td>
            <td><?php echo $row['user_name']; ?></td>
            <td><?php echo $row['lesson_status']; ?></td>
            <td><?php echo $row['credit']; ?></td>
            <td><?php echo $row['total_time']; ?></td>
        </tr>
        <?php } }else{ ?>
            <tr><td colspan="5">No records found.</td></tr>
        <?php } ?>
    </table>
    <input type="submit" class="btn btn-danger" name="bulk_update_submit" value="Done"/>
</form>
</body>

</html>

dbConfig.php:

<?php
$dbHost = 'localhost';
$dbUser = 'root';
$dbPass = null;
$dbName = 'remi';
$conn = mysqli_connect($dbHost,$dbUser,$dbPass,$dbName);
if(!$conn){
    die("Database connection failed: " . mysqli_connect_error());
}
?>