关于php:php实现无限级分类

//有限级分类$list = array(    array('id' => 1, 'pid' => 0, 'name' => '河北省'),    array('id' => 2, 'pid' => 0, 'name' => '北京市'),    array('id' => 3, 'pid' => 1, 'name' => '邯郸市'),    array('id' => 4, 'pid' => 2, 'name' => '朝阳区'),    array('id' => 5, 'pid' => 2, 'name' => '通州区'),    array('id' => 6, 'pid' => 4, 'name' => '望京'),    array('id' => 7, 'pid' => 4, 'name' => '酒仙桥'),    array('id' => 8, 'pid' => 3, 'name' => '永年区'),    array('id' => 9, 'pid' => 1, 'name' => '武安市'),    array('id' => 10, 'pid' => 7, 'name' => '三街坊'),);//传值援用function list_to_tree($list,$id='id',$pid='pid',$child='_child',$root=0){    $tree = [];    if (is_array($list))    {        //创立基于主键的数组援用        $refer = [];        foreach ($list as $k=>$v)        {            $refer[$v[$id]]=&$list[$k];        }        foreach ($list as $k=>$v)        {            $parent_id = $v[$pid];            if ($root==$parent_id)            {                $tree[]=&$list[$k];            }            else            {                if (isset($refer[$parent_id]))                {                    $parent = &$refer[$parent_id];                    $parent[$child][]=&$list[$k];                }            }        }    }    return $tree;}

//递归function getTree($data, $pId){    $tree = array();    foreach($data as $k => $v)    {        if($v['pid'] == $pId)        {        //父亲找到儿子            $v['_child'] = getTree($data, $v['id']);            $tree[] = $v;            unset($data[$k]);        }    }    return $tree;}