Online PHP editor | output for 97S5I

<?php // use ORDER BY belongs_to DESC, id ASC ... or usort() to prepare result set $resultset = [ ['id' => '6a', 'belongs_to' => '5a'], ['id' => '5a', 'belongs_to' => '3a'], ['id' => '8a', 'belongs_to' => '3a'], ['id' => '3a', 'belongs_to' => '1a'], ['id' => '1a', 'belongs_to' => null], ['id' => '2a', 'belongs_to' => null], ['id' => '4a', 'belongs_to' => null], ['id' => '7a', 'belongs_to' => null] ]; foreach ($resultset as $index1 => &$row1) { // make input array modifiable by reference (not working with a copy) if ($row1['belongs_to']) { // original belongs_to value is not null (not a top-level parent) foreach ($resultset as $index2 => $row2) { // search for targeted parent if ($row2['id'] == $row1['belongs_to']) { // parent found $resultset[$index2]['children'][] = [$row1['id'] => $row1['children'] ?? []]; // store original row as child unset($resultset[$index1]); // remove original row (no reason to iterate it again in outer loop) break; // halt inner loop (no reason to iterate further) } } } else { // original belongs_to value is null (top-level parent) $output[$row1['id']] = $row1['children'] ?? []; // store children to top } } var_export($output);