Javascript to PHP updating database problems [closed]

I am making a little website game for class and we have to link options to a form that sends the data to a database through PHP. I have managed to link the database fine for all of the aspects of the game except upating the highscore.

Here is the JS code:

function new_score(score) {

    // set up an XMLHttpRequest object
    var connect = new XMLHttpRequest();
    connect.responseType = "json";

    // run server side query
    var name = document.getElementById("lookup_name_selector").value;

    var query ="update_highscore.php?name=" + name + "&highscore=" + score;
    connect.open("GET", query);

    connect.send();
}

and here is the php file:

require 'get_db_handle_path.php';
require GET_DB_HANDLE_PATH."/get_db_handle.php";

$conn = get_db_handle();

// prepared statements help prevent SQL injection attacks
// if you have to run them lots of times, they're also faster
$stmt = $conn -> prepare();
$stmt = $conn -> 
   prepare("UPDATE favourites SET highscore=:score WHERE name=:name".
           "values (:name, :score)");
$stmt -> bindParam(':name', $_GET['name']);
$stmt -> bindParam(':score', $_GET['highscore']);

$stmt -> execute();

// no need for a loop, since there is at most one result 
// 'name' is primary key for the table favourites

echo json_encode($stmt -> fetch(PDO::FETCH_ASSOC));

// remove all references to the connection
$stmt = null;
$conn = null;

thanks in advanced!