Troubles with ajax. How to convert javascript variable to php variable?

I want to convert javascript variable to php variable to use it in sql query,but it doesn't work.

html code :

<select id = "dep_ID" name = "dep_ID" onchange="myFunction()">

javascript code in the same file:

<script>
    function myFunction(){

        var xo = document.getElementById("dep_ID").value;
        document.getElementById("demo").innerHTML = "You selected: " + xo;

        $.ajax({
            url: 'insert.php',                
            data: {duration: xo},
            type: 'POST',
            success: function(data) {
            alert(data);
            document.getElementById("demo").innerHTML = "You selected: " + <?php echo @$duration;?>;
            }
        });
    }
</script>

insert.php code:

<?php
$duration = $_POST['xo'];
return $duration ;
?>

I expect the output of dep_ID variable, but i get nothing.