EasyTree php tree build algorithm

EasyTree.php

<?php
/**
 * 1. This Algorithm can help you build tree as a nested php array from array like ('id' => x, 'parent_id' => y)
 * 2. Algorithm speed = {O(n log n) + О(N)}
 */
class EasyTree
{
    public static 
        $idKeyName = 'id', $parentIdKeyName = 'parent_id', $childrenKeyName = 'children';

    protected static function operator($a, $b) 
    {
        return ($a[self::$parentIdKeyName] === $b[self::$parentIdKeyName]) ? 
            0 : (($a[self::$parentIdKeyName] < $b[self::$parentIdKeyName]) ? -1 : 1);
    }

    /**
     * @return array
     */
    public static function build(array $a) 
    {
        $tree = $links = array();
        usort($a, "self::operator");
        $acmeId = $a[0][self::$parentIdKeyName];
        for ($n = count($a), $i = 0; $i < $n; $i++) 
        {
            $id = $a[$i][self::$idKeyName];
            $parentId = $a[$i][self::$parentIdKeyName];
            if ($parentId === $acmeId) 
            {
                $tree[$id] = $a[$i];
                $links[$id] = &$tree[$id];
            }
            else
            {
                if (!isset($links[$parentId][self::$childrenKeyName]))
                    $links[$parentId][self::$childrenKeyName] = array();
                $links[$parentId][self::$childrenKeyName][$id] = $a[$i];
                $links[$id] = &$links[$parentId][self::$childrenKeyName][$id];
            }
        }

        return $tree;
    }
}

/*
print_r(
    EasyTree::build(array(
            array('id' => 1, 'parent_id' => 0),
            array('id' => 3, 'parent_id' => 1),
            array('id' => 2, 'parent_id' => 1),
            array('id' => 8, 'parent_id' => 0),
            array('id' => 4, 'parent_id' => 0),
            array('id' => 5, 'parent_id' => 0),
            array('id' => 6, 'parent_id' => 0),
            array('id' => 7, 'parent_id' => 0),
            array('id' => 11, 'parent_id' => 7),
            array('id' => 9, 'parent_id' => 0),
            array('id' => 10, 'parent_id' => 7),
            array('id' => 12, 'parent_id' => 7),
            array('id' => 13, 'parent_id' => 7),
            array('id' => 14, 'parent_id' => 10),
            array('id' => 15, 'parent_id' => 10),
            array('id' => 16, 'parent_id' => 15),
            array('id' => 17, 'parent_id' => 16),
            array('id' => 18, 'parent_id' => 17),
    ))
);
*/

This only works if the parent_ids are guaranteed to be smaller than the child_ids.
If you have a tree that may contain eg. array('id' => 10, 'parent_id' => 99), EasyTree will loose those childs.