Алексей Иванофф

$grouped = [];

foreach ($arr as $n) {
  $grouped[$n['category_id']][] = $n['text'];
}

foreach ($grouped as $key => $values) {
  echo "
    <fieldset>
      <legend>$key</legend>".
      implode('', array_map(fn($n) => "<div>$n</div>", $values))."
    </fieldset>";
}