Suppose that I have two arrays, and I want to add the corresponding elements.

```
$v_4 = [
    [1, 2, 3], 
    [4, 5, 6],
];
$v_5 = [
    [7, 8, 9], 
    [10, 11, 12], 
];

```

How should I construct an addition function to add across these arrays to get:

```
[
    [8, 10, 12], 
    [14, 16, 18],
]

```

I know I need to utilise [`array_map`](http://php.net/manual/de/function.array-map.php) somehow, but I am unsure how to proceed in this multidimensional case.