使用ajax更新数据库值

Question

I'm trying to update a database value each time complete() occurs. The following code is located in a file called 'user.php' and $u is globally defined elsewhere in the file. The value pT simply doesn't update when complete() is ran. Thanks in advance.

<?php
    if (isset($_POST['pT'])){
        $pT = $_POST['pT'];

        $sql = "UPDATE progress SET progressbar='$pT' WHERE username='$u'";
        $query = mysqli_query($conn, $sql);

        exit();
    }
?>

var pT = 0;

function complete(){

    pT = pT + 25;

    $.ajax({
        url: 'user.php',
        type: 'POST',
        data: {'pT': pT},
        success: function() {}
    });

}