How to return data to the front desk in PHP

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

<?php
$host="localhost";
$username="root";
$password="root";
$dbName="baixing";
$port=3306;
$conn=new mysqli($host,$username,$password,$dbName,$port);
if(!$conn){
    die("error:".$conn->connect_error);
}
//设置查询结果的编码，一定要放在query之前
$conn->query("SET NAMES &#39;UTF8&#39;");
$result=$conn->query("select * from hotgoods");
//$conn->query()获取的是二进制
//将查询的结果集封装到一个数组里
$css=$result->fetch_all();
//以json的格式发送ajax的success中由data接收
echo json_encode($css);
$conn->close();