php问题，脑子不行，帮个忙[2.0]

Array(
    [0] => Array
        (
            [created_at] => 2016-08
            [a] => 31900.00
        )
    [1] => Array
        (
            [created_at] => 2016-09
            [a] => 45400.00
        )
    [2] => Array
        (
            [created_at] => 2016-10
            [a] => 69489.00
        )
）

Array
(
    [0] => Array
        (
            [created_at] => 2016-08
            [b] => 21900.00
        )
    [1] => Array
        (
            [created_at] => 2016-09
            [b] => 25400.00
        )
）

怎么合并成下面数组啊

Array
(
    [0] => Array
        (
            [created_at] => 2016-08
            [a] => 31900.00
            [b] => 21900.00
        )
    [1] => Array
        (
            [created_at] => 2016-09
            [a] => 45400.00
            [b] => 25400.00
        )
    [2] => Array
        (
            [created_at] => 2016-10
            [a] => 69489.00
            [b] => 
        )
）

结果 :
以下代码可以满足你的要求，尽管这个要求有点奇怪
从常规索引数组到关联数组，没有想到好办法，只能循环了。
很难判断算法在大数据量时的优缺点