PHP Splitting an Array into two arrays - keys array and values array

Question

Is there an easy way to split an array into two arrays, one consisting of all the keys and the other consisting of all the values? This would be a reverse to the action of

Answer 1

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 the "call_user_func_array" function:

$array_comb  = function($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 arrays-of-arrays, where the outer array contains 2 elements, both inner arrays are of equal length and the first inner array only contains integers and strings.