$arr = [
['id' => 1,
            'menu_collapse' => '超级菜单',
            'menu_items' => '写文章',
            'cid' => 1,
         
        ],

   [ 'id' => 2,
            'menu_collapse' => '维护菜单',
            'menu_items' => '管理数据',
            'cid' => 2,
         
        ],

    [  'id' => 3,
            'menu_collapse' => '维护菜单',
            'menu_items' => '管理用户',
            'cid' => 2,
         
        ]
];

$result=[];
$res=[];
foreach($arr as $k=>$v){
    $result[$v['cid']][]=$v;
}

foreach($result as $key=>$value){
  if(count($value) == 1) {
    $res[$key]=$value[0];
  } else{
    $res[$key]['new']=$value;
  }
}

echo '<pre>';print_r(array_values($res));
/*
Array
(
    [0] => Array
        (
            [id] => 1
            [menu_collapse] => 超级菜单
            [menu_items] => 写文章
            [cid] => 1
        )

    [1] => Array
        (
            [new] => Array
                (
                    [0] => Array
                        (
                            [id] => 2
                            [menu_collapse] => 维护菜单
                            [menu_items] => 管理数据
                            [cid] => 2
                        )

                    [1] => Array
                        (
                            [id] => 3
                            [menu_collapse] => 维护菜单
                            [menu_items] => 管理用户
                            [cid] => 2
                        )

                )

        )

)
*/

你只能遍历数组来处理了。