Jquery sortable PDO update order - php

Question

I'm trying to create a sortable list that updates by PDO (PHP Data Objects).
I've tried to work something out, although it seems like it doesn't want to update, and i'm unsure where I've gone wrong. Drag works fine, data seems to work correct. Just doesn't update it to the database.
Main file with JQuery
<ul id="sortable">
<?php
$sql = "SELECT * FROM ".$prefix."question WHERE enabled = 1 ORDER BY sortby ASC";
$stm = $dbh->prepare($sql);
$stm->execute();
$u = $stm->fetchAll();
$count = 0;
foreach ($u as $sup) {
?>
<li id="item-<?php echo $sup['id']; ?>"><?php echo $sup['title']; ?></li>
<?php } ?>
</ul>
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script>
$('#sortable').sortable({
axis: 'y',
update: function (event, ui) {
var data = $(this).sortable('serialize');
// POST to server using $.post or $.ajax
$.ajax({
data: data,
type: 'POST',
url: 'order.php'
});
}
});
</script>
PHP File [order.php] (Connection to Database is fine and works correctly)
<?php
ob_start();
session_start();
$admin = true;
require "inc/config.php";
$i = 0;
foreach ($_POST['item'] as $value) {
// Execute statement:
// UPDATE [Table] SET [Position] = $i WHERE [EntityId] = $value
$sql = $dbh->prepare("UPDATE ".$prefix."question SET sortby='".$i."' WHERE id=1");
$sql->execute();
$i++;
}
?>
If anybody could see what my problem is, I'd really appreciate it.

Answer 1

Was able to fix this. Was actually an issue with how I was getting the config.php for the database.
Rest of the code works.

Answer 2

Since you're calling files over ajax you won't be able to see any errors in the order.php, and to do that you need to change your ajax call to the following:
$.ajax({
data: data,
type: 'POST',
url: 'order.php',
success: function(data){
alert(data); // for testing
}
});
Now the problem i think your having is using a pdo prepare statement without supplying any data for the statement, you need to supply data to the execute method, use binding or use the query method instead.
$sql = $dbh->prepare("UPDATE ".$prefix."question SET sortby= ? WHERE id=?");
$sql->execute([$i,1]);
Or you can use the query statement if you have other means of sanitizing your data:
$sql = $dbh->query("UPDATE ".$prefix."question SET sortby='".$i."' WHERE id=1");