function tree($data)
{
    $res = [];
    $i =1;
    foreach ($data as $key => $value) {
        echo "\n第{$i}次\n"; 
        var_dump(isset($data[$value['pid']]));
        if (isset($data[$value['pid']])) {
            $data[$value['pid']]['child'][] = &$data[$key];
            var_dump($data);
            echo "\n";
        }else{
            $res[] = &$data[$key];
            var_dump($res);
            echo "\n"; 
        }
        $i++;
    }
    return $res;
}