if (isset($_POST['submit'])) {

    foreach ($_POST['todelete'] as $delete_id) { //这里是循环遍历这个数组 todelete 每一个值被设置成 $delete_id

      $query = "DELETE FROM email_list WHERE id = $delete_id";

      mysqli_query($dbc, $query)

        or die('Error querying database.');

    } 

    echo 'Customer(s) removed.<br />';

  }

  // Display the customer rows with checkboxes for deleting

  $query = "SELECT * FROM email_list";

  $result = mysqli_query($dbc, $query);

  while ($row = mysqli_fetch_array($result)) {

    // todelete[]是一个数组 能够把checkbox中选中的放到这个数组中

    echo '<input type="checkbox" value="' . $row['id'] . '" name="todelete[]" />';

    echo $row['first_name'];

    echo ' ' . $row['last_name'];

    echo ' ' . $row['email'];

    echo '<br />';

  }