I have nested arrays that do not have keys. I want to add keys in a particular order. What is a clean way to do this? 

Start with this array. It is only indexed by position.

```
[0] => (
    [0] => Tyler
    [1] => Durden
    [2] => 05/07/1985
)
[1] => (
    [0] => Edward
    [1] => Norton
    [2] => 03/21/1988
)

```

Now apply these keys in order:

```
['first_name'] =>
['last_name'] =>
['birthday'] =>

```

Final array:

```
[0] => (
   ['first_name'] => Tyler
   ['last_name'] => Durden
   ['birthday'] => 05/071985
)
[1] => (
    ['first_name'] => Edward
    ['last_name'] => Norton
    ['birthday'] => 03/21/1988
)

```

Bonus upvotes if your code allows for any key structure, instead of being hard-coded!