php连接数据库 搜索数据形成数组,转为字符串

<?php  //连接数据库

$db = new mysqli('localhost','root','root','z_0222');

!mysqli_connect_error() or die('数据库连接失败');

$db->query('set names utf8');

//搜索语句

$sql = "select * from student";
$res = $db->query($sql);

//转为数组

$arr = $res->fetch_all();

//数组转为字符串

$str = '';
foreach ($arr as $v){
$str .= implode(',',$v).'^';  //以'^'分隔
}

$str = substr($str,0,strlen($str)-1);  //删除最后逗号

echo $str;