mysql数据库中有数据表但php无法引用

<?php
//1.连接数据库
try{
$pdo=new PDO("mysql:127.0.0.1;dbname=myapp","root","");

   }catch(PDOException $e){
       die("数据连接失败".$e->getMessage());
   }
    //2.执行SQL查询，并解析与遍历
    $sql="SELECT  * FROM  users";
    $result=mysql_query($sql);
    if(is_array($sql)) {
        foreach ($pdo->query($sql) as $row) {
            echo "<tr>";
            echo "<td>{$row['id']}</td>";
            echo "<td>{$row['name']}</td>";
            echo "<td>{$row['age']}</td>";
            echo "<td>{$row['classid']}</td>";
            echo "<td>操作</td>";
            echo "</tr>";
        }
    }

?>