gistfile1.php

<?php
$result = [];

foreach(db::test()->query('SELECT `id`,`title`, `parent_id` FROM `table` GROUP BY `parent_id` ORDER BY `parent_id` ASC')->fetchAll() as $item) {
  if($item['parent_id'] == 0) {
    if(!isset($result[$item['id']])) {
      $result[$item['id']] = $item;
    }
  } else {
    if(!isset($result[$item['id']]['child'])) $result[$item['id']]['child'] = [];
    $result[$item['id']]['child'][] = $item;
  }
}

echo json_encode(array_values($result));