Bungkus elemen array dalam div berdasarkan nilai yang sama

$rows = $users->fetchAll(PDO::FETCH_ASSOC);
$arrayByGroup = array();

$id = null;
foreach ($rows as $r) {
  if($id != $r['id_group']) {
    if (!is_null($id)) {
      echo '</div>';
    }
    $id = $r['id_group'];
    echo '<div class="id_group_' . $id . '">';
  }
  echo "<div>".$r["comments"]."<br>Written by ".$r["name"]."</div>";

}
echo '</div>'

Edit dibuat berdasarkan wawasan Matt Barrett.