error when deleting from the header PHP PDO

Question

test1:

if (isset($_POST['delete_submit'])) {
    $sql = "DELETE FROM `users_item` WHERE id=?";
    $query = $dbh->prepare($sql);
    $query->execute([$id]);
    header('Location: '. $_SERVER['HTTP_REFERER']);
}
 <?php foreach ($result as $value) { ?>
                    <tr>
                        <td><?=$value['id'] ?></td>
                        <td><?=$value['username'] ?></td>
           
                        <!-- <td></td> -->
                        <td>
                            <a href="?edit=<?=$value['id'] ?>" class="btn btn-success btn-sm" data-toggle="modal"
                                data-target="#editModal<?=$value['id'] ?>"><i class="fa fa-edit"></i></a>
                            <a href="?delete=<?=$value['id'] ?>" class="btn btn-danger btn-sm" data-toggle="modal"
                                data-target="#deleteModal<?=$value['id'] ?>"><i class="fa fa-trash"></i></a>
                                <?php require 'test2';?>
                        </td>
                    </tr> <?php } ?>

test2:

<!-- DELETE MODAL -->
<div class="modal fade" id="deleteModal<?=$value['id'] ?>" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog modal-dialog-centered" role="document">
    <div class="modal-content shadow">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Delete: № <?=$value['id'];?></h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Closed</button>
        <form action="?id=<?=$value['id'] ?>" method="post">
            <button type="submit" name="delete_submit" class="btn btn-danger">Delete</button>
        </form>
      </div>
    </div>
  </div>
</div>

error: Warning: Cannot modify header information - headers already sent by (output started at test1.php:4) in test2.php on line 37, whats wrong?