无限极分类

    function generateTree($items){
        $tree = array();
        foreach($items as $item){
            if(isset($items[$item['pid']])){
                $items[$item['pid']]['son'][] = &$items[$item['id']];
            }else{
                $tree[] = &$items[$item['id']];
            }
        }
        return $tree;
    }
    $items = array(
        1 => array('id' => 1, 'pid' => 0, 'name' => '安徽省'),
        2 => array('id' => 2, 'pid' => 0, 'name' => '浙江省'),
        3 => array('id' => 3, 'pid' => 1, 'name' => '合肥市'),
        4 => array('id' => 4, 'pid' => 3, 'name' => '长丰县'),
        5 => array('id' => 5, 'pid' => 1, 'name' => '安庆市'),
    );
    print_r(generateTree($items));

    Array
    (
        [0] => Array
            (
                [id] => 1
                [pid] => 0
                [name] => 安徽省
                [son] => Array
                    (
                        [0] => Array
                            (
                                [id] => 3
                                [pid] => 1
                                [name] => 合肥市
                                [son] => Array
                                    (
                                        [0] => Array
                                            (
                                                [id] => 4
                                                [pid] => 3
                                                [name] => 长丰县
                                            )
     
                                    )
     
                            )
     
                        [1] => Array
                            (
                                [id] => 5
                                [pid] => 1
                                [name] => 安庆市
                            )
     
                    )
     
            )
     
        [1] => Array
            (
                [id] => 2
                [pid] => 0
                [name] => 浙江省
            )
     
    )

上面生成树方法还可以精简到5行：

    function generateTree($items){
        foreach($items as $item)
            $items[$item['pid']]['son'][$item['id']] = &$items[$item['id']];
        return isset($items[0]['son']) ? $items[0]['son'] : array();
    }

上面这种无限极分类数据树形结构化的方法值得借鉴。但是我觉得这段代码实际用途并不明显啊，你想取出格式化的树形数据还是要递归啊：

    /**
     * 如何取数据格式化的树形数据
     * @blog<http://www.phpddt.com>
     */
    $tree = generateTree($items);
    function getTreeData($tree){
        foreach($tree as $t){
            echo $t['name'].'<br>';
            if(isset($t['son'])){
                getTreeData($t['son']);
            }
        }
    }
    getTreeData($tree);