php新手请教一个问题；
原数组结构如下，原数组是按ads_id，time进行排序；
现在的需求是根据二维数组的num总和重新进行排序

Array ( [5] => Array ( [0] => Array ( [num] => 840 [time] => 2015-10 [ads_id] => 5 ) [1] => Array ( [num] => 684 [time] => 2015-11 [ads_id] => 5 ) ) [6] => Array ( [0] => Array ( [num] => 23 [time] => 2015-10 [ads_id] => 6 ) [1] => Array ( [num] => 37 [time] => 2015-11 [ads_id] => 6 ) ) [7] => Array ( [0] => Array ( [num] => 280 [time] => 2015-10 [ads_id] => 7 ) [1] => Array ( [num] => 343 [time] => 2015-11 [ads_id] => 7 ) ) )

最终实现排序如下：

Array ( [5] => Array ( [0] => Array ( [num] => 840 [time] => 2015-10 [ads_id] => 5 ) [1] => Array ( [num] => 684 [time] => 2015-11 [ads_id] => 5 ) ) [7] => Array ( [0] => Array ( [num] => 280 [time] => 2015-10 [ads_id] => 7 ) [1] => Array ( [num] => 343 [time] => 2015-11 [ads_id] => 7 ) ) [6] => Array ( [0] => Array ( [num] => 23 [time] => 2015-10 [ads_id] => 6 ) [1] => Array ( [num] => 37 [time] => 2015-11 [ads_id] => 6 ) ) )