使用ajax更新数据库值

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() {}
    });

}

1. split the files

   user.php
   _______
   <?php
   if (isset($_POST['pT'])){
       $pT = $_POST['pT'];
   
       $sql = "UPDATE progress SET progressbar='$pT' WHERE username='$u'";
       $query = mysqli_query($conn, $sql);
   
       exit();
   }
   ?>
   

2. then make sure your javascript/jquery ajax isn't malformed

   somewherelse.html
   ____________
   var pT = 0;
   
   function complete(){
   
     pT = pT + 25;
   
     $.ajax({
       url: 'user.php',
       type: 'POST',
       data: 'pT='+ pT,
       success: function() {
           alert('done');
       }
     });
   
   }