PHP中表单提交数据后保留上一次的数据怎么做

Question

<form action="" method="post">
<td>编号：<input class="sort" type="text" name="classroomId" value="<?php if(isset($_POST['classroomId'])){echo $_POST['classroomId'];}?>"  /></td>
        
        <input type="submit" name="chaxun" value="查询" />

        </form>

报错：Notice: Undefined index: user in