closure-iterator.php

<?php

$list = $_pdo->getAll('SELECT s1.`id` `id` FROM cat_section s1
      LEFT JOIN cat_section s2 ON s1.`parent_id` = s2.`id`
      WHERE s1.`parent_id` <> 0 AND s2.`id` IS NULL', [], \PDO::FETCH_COLUMN);

$iter = function($sid)
use(&$iter, $_pdo)
{
  foreach($_pdo->getAll('SELECT s1.`id` `id` FROM cat_section s1
    WHERE s1.`parent_id` = ?', [$sid], \PDO::FETCH_COLUMN) as $id
  )
  {
    yield $id;
    foreach($iter($id) as $cid)
    {
      yield $cid;
    }
  }
};

$data = $list;
foreach($list as $sid)
{
  foreach($iter($sid) as $id)
  {
    $data[] = $id;
  }
}