多級分類PHP，有效優化

/**
 * 數組轉樹
 * @param type $list
 * @param type $root
 * @param type $pk
 * @param type $pid
 * @param type $child
 * @return type
 */
function list_to_tree($list, $root = 0, $pk = 'id', $pid = 'parentid', $child = '_child') {
    // 創建Tree
    $tree = array();
    if (is_array($list)) {
        // 創建基於主鍵的數組引用
        $refer = array();
        foreach ($list as $key => $data) {
            $refer[$data[$pk]] = &$list[$key];
        }
        foreach ($list as $key => $data) {
            // 判斷是否存在parent
            $parentId = 0;
            if (isset($data[$pid])) {
                $parentId = $data[$pid];
            }
            if ((string) $root == $parentId) {
                $tree[] = &$list[$key];
            } else {
                if (isset($refer[$parentId])) {
                    $parent = &$refer[$parentId];
                    $parent[$child][] = &$list[$key];
                }
            }
        }
    }
    return $tree;
}