http://creative-punch.net/2014/01/creating-nested-array-items-parent-ids/
http://stackoverflow.com/questions/9996427/getting-a-list-of-children-from-an-array-with-parents-without-recursion-in-php

// Create a nested array.
function makeNested($source) {
	$nested = array();

	foreach ( $source as &$s ) {
		if ( is_null($s['parent_id']) ) {
			// no parent_id so we put it in the root of the array
			$nested[] = &$s;
		}
		else {
			$pid = $s['parent_id'];
			if ( isset($source[$pid]) ) {
				// If the parent ID exists in the source array
				// we add it to the 'children' array of the parent after initializing it.

				if ( !isset($source[$pid]['children']) ) {
					$source[$pid]['children'] = array();
				}

				$source[$pid]['children'][] = &$s;
			}
		}
	}
	return $nested;
}