如何使用PHP将JQuery UI可排序表位置保存到MySQL数据库？

i make an php web application for manage site products

i wanted to use jquery sortable for sorting the product position i use this method:

var fixHelper = function(e, ui) {
    ui.children().each(function() {
        $(this).width($(this).width());
    });
    return ui;
};
$( "#sort tbody" ).sortable({
    placeholder: "ui-state-highlight", axis: 'y', helper: fixHelper,
    update: function (event, ui) {
        var data = $(this).sortable('serialize');

        // POST to server using $.post or $.ajax
        $.ajax({
            data: data,
            type: 'POST',
            url: '/www/order.php'
        });
    }
});
$( "#sort tbody" ).disableSelection();

and in order.php i use this

<?php
include("db.php");

$i = 0;

foreach ($_POST['item'] as $value) {
    Execute statement:
          mysql_query(UPDATE cp_login SET [Position] = $i WHERE [EntityId] = $value);
    $i++;
}

include("db-close.php");
?>

this is not working what should i type in [position] section and [EntityId] section i could not understand please give me fixed code with explanation .

thank you