从javascript将数据保存到db

Question

如何将javascript变量保存到MYSQL数据库中？特别是我希望在游戏结束时保存游戏的分数。请参阅以下代码：

function lastScore(num){
    document.getElementById('lastscore').textContent = num;
}


function saveScore(){
    xmlhttp = new XMLHttpRequest();
    xmlhttp.open("GET","saveScore.php?s="+player.score,false);
    xmlhttp.send(null);
    document.getElementById("endgame").innerHTML = xmlhttp.responseText;
}

PHP

<?php
include('server.php');

$s =  $_GET['s'];
$db = mysqli_connect( ...MY DATA...);

$query = "INSERT INTO scores (name, score) VALUES ('$username' , '$s')";
mysqli_query($db, $query);
?>

你能告诉我什么是错的吗？我该怎么做才能将player.score作为参数插入到MYSQL数据库中？感谢