求助，php输入信息查询数据库为什么没返回值呢

<?php
$userid = trim($_POST['name']);
$dn = trim($_POST['vipid']);
$con = mysqli_connect("127.0.0.1","root","root2020","ucd_dn");
if (!$con)
  {
  die('Could not connect: ' . mysqli_error());
  }
$a = mysqli_select_db( $con,"ucd_dn");
$sql = "select * from ucddn where remark = '$userid' and extension = '$dn' ";
$result = mysqli_query($con,$sql);
while($row = mysqli_fetch_array($result))
  {
  echo $row['extension'] . "   工号:" . $row['remark'];
  echo "<br />";
  }
mysqli_close($con);
?>

phpstudy搭的php7.3.4环境，运行以后页面是空的，空页面，查看源码也没有任何内容，没有显示结果，这是为啥啊，麻烦指点一下

数据库信息如下图：