# Inverse of PHP’s array_combine

Another shameless re-post of an
answer I gave on Stack Overflow. This time it’s about a function to
split an `array`

into its
keys and its values, ie. an inverse of `array_combine`

.

Unfortunately there is no built-in inverse of `array_combine`

.
There is also no way to define one, since `array_combine`

expects multiple parameters and we can’t return multiple values from a
function.

We can construct an alternative to `array_combine`

which takes a single argument: the `array`

of keys and
the `array`

of
values wrapped up together in another `array`

. This
transformation is called “uncurrying” and is performed by `call_user_func_array`

:

`$array_comb = function($arr) { return call_user_func_array('array_combine', $arr); };`

This alternative function does have an inverse:

`$array_split = function($arr) { return array(array_keys($arr), array_values($arr)); };`

If we define function composition:

```
$compose = function($f, $g) {
return function($x) use ($f, $g) { return $f($g($x)); };
; }
```

Then the following functions are all (extensionally) equal, ie. they all return their argument unchanged:

```
$identity = function($x) { return $x; };
$left_inverse = $compose($array_split, $array_comb); // Split then combine
$right_inverse = $compose($array_comb, $array_split); // Combine then split
```

Note that they accept different argument types though:

`$identity`

will work on anything.`$left_inverse`

will work on any`array`

.`$right_inverse`

will work on`array`

s-of-`array`

s, where the outer`array`

contains 2 elements, both inner`array`

s are of equal length and the first inner`array`

only contains`int`

egers and`string`

s.