Ошибка в приложении. "Invalid argument supplied for foreach()"

<?php
 
$pdo = new PDO('mysql:host=127.0.0.1;dbname=projectbd34', 'projectbd34', 'Zxyd4L8m');
 
 
 
switch (@$_GET['action'])
{
  case 'add':
    $clients = $pdo->query('SELECT * FROM `clients`');
    $sellers = $pdo->query('SELECT * FROM `sellers`');
    $products = $pdo->query('SELECT * FROM `products`');
 
    $url = '/orders.php?action=create';
    include 'forms/order.php';
  break;
 
  case 'create':
    $sql = $pdo->prepare('INSERT INTO `orders` (`clients_id`, `sellers_id`, `date`, `time` `products_id`) VALUES (:subject_id, :student_id, :date, :mark, :products_id)');
    $sql->execute([
      ':clients_id' => (int)$_POST['clients_id'], 
      ':sellers_id' => (int)$_POST['sellers_id'], 
      ':date' => $_POST['date'], 
      ':time' => (int)$_POST['time'],
      ':products_id' => (int)$_POST['products_id']
    ]);
    echo 'Успешно добавлено!<br><a href="/orders.php">Успеваемость</a>';
  break;
  
  case 'edit':
    $sql = $pdo->prepare('SELECT * FROM `orders` WHERE `id` = :id');
    $sql->execute([':id' => $_GET['id']]);
    $mark = $sql->fetch();
 
    $clients = $pdo->query('SELECT * FROM `clients`');
    $sellers = $pdo->query('SELECT * FROM `sellers`');
    $products = $pdo->query('SELECT * FROM `products`');
 
    $url = '/orders.php?action=update&id=' . $_GET['id'];
    include 'forms/order.php';
  break;
  
  case 'update':
    $sql = $pdo->prepare('UPDATE `orders` SET `clients_id` = :clients_id, `sellers_id` = :sellers_id, `date` = :date, `time` = :time, `products_id` = :products_id WHERE `id` = :id LIMIT 1');
    $sql->execute([
      ':id' => $_GET['id'],
      ':clients_id' => (int)$_POST['clients_id'], 
      ':sellers_id' => (int)$_POST['sellers_id'], 
      ':date' => $_POST['date'], 
      ':time' => (int)$_POST['time'],
      ':products_id' => (int)$_POST['products_id']
    ]);
    echo 'Оценка успешно обновлена!<br><a href="/orders.php">Успеваемость</a>';
  break;
 
  case 'delete':
    $sql = $pdo->prepare('DELETE FROM `orders` WHERE `id` = :id LIMIT 1');
    $sql->execute([':id' => $_GET['id']]);
    echo 'Удалено!<br><a href="/orders.php">Успеваемость</a>';
  break;
 
  default:
 
    echo '[ <a href="/">Вернуться на главную</a> ]<hr>';
 
    echo '<a href="/orders.php?action=add">Добавить оценку</a><br>';
 
    $orders = $pdo->query('
      SELECT 
        `m`.`id`, 
        `m`.`date`, 
        `m`.`mark`, 
        CONCAT_WS(" ", `s`.`firstname`, `s`.`lastname`) `orders`, 
        `j`.`name` `clients_name` 
      FROM 
        `orders` `m`, 
        `students` `s`, 
        `subjects` `j` 
      WHERE 
        `m`.`student_id` = `s`.`id` 
        AND
        `m`.`subject_id` = `j`.`id`
      ORDER BY `date` DESC');
 
 
    echo '<table border="1" cellspacing="0">';
 
    echo '<tr>';
    echo '<th>ID</th>';
    echo '<th>Клиент</th>';
    echo '<th>Продавец</th>';
    echo '<th>Дата</th>';
    echo '<th>Время</th>';
    echo '<th>Товар</th>';
    echo '<th>&nbsp;</th>';
    echo '</tr>';
 
    foreach ($orders as $order)
    {
      echo '<tr>';
      echo '<td>' . $order['id'] . '</td> ' 
      . '<td>' . $order['clients_id'] . '</td> ' 
      . '<td>' . $order['sellers_id'] . '</td> ' 
      . '<td>' . $order['date'] . '</td> ' 
      . '<td>' . $order['time'] . '</td> ' 
      . '<td>' . $order['products_id'] . '</td> ' 
      . '<td><a href="/orders.php?action=edit&id=' . $order['id'] . '">ред.</a> <a href="/orders.php?action=delete&id=' . $order['id'] . '">уд.</a></td>';
      echo '</tr>';
 
    }
    echo '</table>';
 
  break;
 
}

foreach ($orders as $order)

var_dump($orders);
while($order = $orders->fetch(\PDO::FETCH_ASSOC))