Convert adjacency list into tree without recursion and second array.

function_v1.php

<?php

function array_to_tree(array $array, $parent_id = 0)
{
    $array = array_combine(array_column($array, 'id'), array_values($array));

    foreach ($array as $k => &$v) {
        if (isset($array[$v['parent_id']])) {
            $array[$v['parent_id']]['children'][$k] = &$v;
        }
        unset($v);
    }

    return array_filter($array, function($v) use ($parent_id) {
        return $v['parent_id'] == $parent_id;
    });
}

function_v2.php

<?php

/**
 * Convert adjacency list into tree without recursion and second array.
 *
 * @param array $array array to convert
 * @param int|null $parent_id start with given parent id
 * @param string $primary_key primary key name
 * @param string $foreign_key foreign key name
 * @return array|null converted array
 */
function array_to_tree(array $array, $parent_id = 0, $primary_key = 'id', $foreign_key = 'parent_id') {
    $first_element = current($array);
    
    if (!is_array($first_element)) {
        trigger_error('array_to_tree expects parameter 1 to be a multi-dimensional array, ' . gettype($first_element) . ' given', E_USER_WARNING);
        return null;
    }

    if (!array_key_exists($primary_key, $first_element)) {
        trigger_error('array_to_tree expects child array to have a ' . $primary_key . ' key', E_USER_WARNING);
        return null;
    }

    if (!array_key_exists($foreign_key, $first_element)) {
        trigger_error('array_to_tree expects child array to have a ' . $foreign_key . ' key', E_USER_WARNING);
        return null;
    }

    $keys = array_map(function($e) use ($primary_key) {
        return $e[$primary_key]; 
    }, $array);
    $values = array_values($array);
    $array = array_combine($keys, $values);
 
    foreach ($array as $k => &$v) {
        if (isset($array[$v[$foreign_key]])) {
            $array[$v[$foreign_key]]['children'][$k] = &$v;
        }
        unset($v);
    }
 
    return array_filter($array, function($v) use ($parent_id, $foreign_key) {
        return $v[$foreign_key] == $parent_id;
    });
}

if PHP < 5.5

Thank you. Saved me lot of time! :)