php 预处理查询参数这样写怎么错误了?

在数据库“表明”查询姓名为“名字”，输出相关a,b两个字段的数据；为什么在bind_result 行出现

Warning: mysqli_stmt::bind_result(): Number of bind variables doesn't match number of fields in prepared statement

而

$stmt->bind_result($res);改成

$stmt->bind_result($a,$b)时就可以成功了，。

为什么$res这个参数传递错误？怎么改呢？

$sql="SELECT a,b FROM 表名 WHERE name=? ";

select("true",$sql,"s","名字",'$a,$b');


function select($isclude,$sql,$s,$value,$res){
if ($isclude=="true"){
require("config.php");
$cn = new mysqli(HOST,USER,PASS,DBNAME);
$cn->query("set names 'utf8'");
}
 $sql=$sql;
 $stmt=$cn->prepare($sql) or die($cn->error);
$stmt->bind_param($s,$value);
$stmt->bind_result($res);
$stmt->execute();
while($stmt->fetch()){
echo $a;
 }
$stmt->free_result();
$stmt->close();
}