php怎么查询mysql并返回json

<?php
//设置编码格式
header("Content-type:application/json;charset=utf-8");   
$servername = "localhost";  
$username = "root";  
$password = "root";  
$dbname = "test";  
class User {
public $id;
public $name;
public $age;
public $sex;
}
//汉字转码
function decodeUnicode($str){
  return preg_replace_callback('/\\u([0-9a-f]{4})/i',
    create_function(
      '$matches',
      'return mb_convert_encoding(pack("H*", $matches[1]), "UTF-8", "UCS-2BE");'
    ),
    $str);
}
$data = array();
// 创建连接  
$con =mysqli_connect($servername, $username, $password, $dbname);  
// 检测连接  
$sql = "SELECT * FROM `user` WHERE 1";  
$result = mysqli_query($con,$sql);  
if($result){
while ($row=mysqli_fetch_array($result,MYSQL_ASSOC)){ 
    $user = new User();
$user->id = $row["id"];
$user->name = $row["name"];
$user->age = $row["age"];
$user->sex = $row["sex"];
$data[]=$user;
}
//打印编码后的json字符串
    $json = json_encode($data);
    echo decodeUnicode($json);
}else{
echo "查询失败";
}
mysqli_close($con);
?>