如果匹配随机任务，请检查给定的答案

嗨，我正在使用此代码生成随机问题：

<?php
session_start(); //this was missing from this file
//connect to db
require_once('db_conn.php');
$id=rand(0,13); //2nd number = highest ID
if ($res = $mysqli->query("SELECT quest FROM table WHERE ID='$id'")){
   $resQ = $res->fetch_assoc(); 
   echo "" . $resQ['quest'] . "";
   $_SESSION['chckanswer'] = $resQ['guest'];
   $res->close();
}
$mysqli->close();
?>

这一次尝试检查给出的答案是否与问题匹配：

<?php
session_start();
//more code
if($stmt = $mysqli->prepare('SELECT quest FROM table WHERE answer=?')){
    $stmt->bind_param("s", $_POST['given_answer']);
    $stmt->execute();
    $stmt->bind_result($quest);
    $stmt->fetch();
        if($_SESSION['chckanswer'] != $quest && !empty($_SESSION['chckanswer'])){
        echo 'wrong';
        exit();
        }
        else{
        echo 'ok';
        }
    $stmt->close();    
}
//more code

但我得到错误未定义索引chckanswer