Ajax MySQL插入无法正常工作

I have the following Javascript code triggered after a game action in order to save current score to a DB using ajax:

            $(oMain).on("save_score", function(evt,iMoney) {

               //  alert ('you won: $'+iMoney);

                 $.ajax({
                 type: "POST",
                 url: 'write.php',
                 data: ({prize:iMoney}),
                 success: function(data) {
                 alert(data);
                 }
             });

             });

and i have the write.php file writing to the DB :

<?php
$dbhost = "localhost";
$dbuser = "root";
$dbpass = "admin";
$dbname = "games";

$prize=$_POST['prize'];

$query="INSERT INTO game_result (user_, amount)
        VALUES('user','$prize')";
$con =  new mysqli($dbhost, $dbuser, $dbpass,$dbname);

$qry_result = $con->query($query);
?>

nevertheless Nothing happens on db side and ajax returns an empty string. What am I doing wrong?