Returns a hierachically nidified array from a linear one. Infinite levels of nesting.

buildhier.php

function buildhier($items, $pk = 'parent_id', $ck = 'id', $subk = 'children') {
	$map = array();
	foreach($items as $k => &$item) {
		$c = $item[$ck];
		$map[$c] = &$item;
	}
	unset($item);
	foreach($items as $item) {
		$p = $item[$pk];
		if(!$p)
			continue;
		$c = $item[$ck];
		if(!isset($map[$p][$subk]))
			$map[$p][$subk] = array();
		$map[$p][$subk][$c] = $map[$c];
		$map[$c]['__rm'] = 1;
		$map[$c] = &$map[$p][$subk][$c];
	}
	foreach($items as $k => $item) {
		if(@$item['__rm'])
			unset($items[$k]);
	}

	return $items;
}