php 查询对应表所有数据

<?php
    header("Content-type:text/html;charset=utf-8");
    $mysqli=new mysqli('localhost','root','');
    mysqli_select_db($mysqli,'mytest');
    $sql="select * from message";
    $res=$mysqli->query($sql);
    $arr=array();
    while ($row=$res->fetch_assoc()) {
        $arr[]=$row;
    }
    $res->free();
    //关闭连接
    $mysqli->close();
    
    echo(json_encode($arr));//这里用echo而不是return
 
?>