Restructure array by parent/child IDs. Recursion?

Question

I have an array of locations. Each of these locations can have child locations. Each of the child locations can also have children, and so on:

$locations = array(
    array("id" => 1,  "parent_id" => 0, "name" => "England"),
    array("id" => 2,  "parent_id" => 0, "name" => "Scotland"),
    array("id" => 3,  "parent_id" => 0, "name" => "Ireland"),
    array("id" => 4,  "parent_id" => 0, "name" => "Wales"),
    array("id" => 5,  "parent_id" => 1, "name" => "East England"),
    array("id" => 6,  "parent_id" => 1, "name" => "London"),
    array("id" => 7,  "parent_id" => 6, "name" => "West London"),
    array("id" => 8,  "parent_id" => 6, "name" => "East London"),
    array("id" => 9,  "parent_id" => 1, "name" => "East Midlands"),
    array("id" => 10, "parent_id" => 9, "name" => "Derbyshire")
);

I want to re-structure this array so that the children are arrays of the parent. Something like this (untested):

$locations =    array("id" => 1, "parent_id" => 0, "name" => "England", "children" => array(
                    array("id" => 5,  "parent_id" => 1, "name" => "East England"),
                    array("id" => 6,  "parent_id" => 1, "name" => "London", "children" => array(
                            array("id" => 7,  "parent_id" => 6, "name" => "West London"),
                            array("id" => 8,  "parent_id" => 6, "name" => "East London")))));

This is so I can then print them out using indents like so:

LOCATIONS

England
- East England
- London
-- West London
-- East London
- East Midlands
-- Derbyshire
Scotland
Ireland
Wales

I have tried several ways, like grouping them by parent ID, but I just can't work out the logic for this, and there may be a better way of doing it (recursion, perhaps?).

Many thanks.

Answer 1

Hi perhaps this will help you, i just wrote it to quickly convert a mysql result containing a parent_id into a usable data hierarchy. Your input array should also work. It's just a couple of lines with two basic loops. No recursion required. Some comments included:

<?php

$max = count($inputArray);
$tree = array();
$flat = array();
// Create a flat hierarchy array of all entries by their id
for ($i = 0; $i < $max; $i++) {
        $n = $inputArray[$i];
        $id = $n['page_id'];
        $flat[$id] = $n;
}
// Then check all those entries by reference
foreach ($flat as $key => &$child) {
        // Add a children array if not already existing
        if (!isset($child['children']))
                $child['children'] = array();

        $id = $child['page_id'];
        $pid = $child['parent_id'];

        // If childs parent id is larger then zero
        if ($pid > 0) {
                // Append it by reference, which means it will reference
                // the same object across different carriers
                $flat[$pid]['children'][] = &$child;
        } else {
                // Otherwise it is zero level, which initiates the tree
                $tree[$id] = &$child;
        }
}

$tree = array_values($tree); // Indices fixed, there we go, use $tree further
?>

So notice the by '& reference' characters. They do all the work allowing the tree to be built from the flat array by pointing to the same objects.