php从数据库读取数据，并以json格式返回数据

$servername = "localhost";
$username = "root";
$password = "root";
$mysqlname = "datatest"; 
$json = '';
$data = array();
class User 
{
public $id;
public $fname;
public $lname;
public $email;
public $password;
}

// 创建连接
$conn = mysqli_connect($servername, $username, $password, $mysqlname);

$sql = "SELECT * FROM userinfo";
$result = $conn->query($sql);

if($result){
//echo "查询成功";
while ($row = mysqli_fetch_array($result,MYSQL_ASSOC))
{
$user = new User();
$user->id = $row["id"];
$user->fname = $row["fname"];
$user->lname = $row["lname"];
$user->email = $row["email"];
$user->password = $row["password"];
$data[]=$user;
}
$json = json_encode($data);//把数据转换为JSON数据.
echo "{".'"user"'.":".$json."}";
}else{
echo "查询失败";
}