PHP RFC: array_change_keys()


This RFC proposes a new core function to simplify the process of changing an array's keys (“re-keying”).

The Problem

PHP only has one existing function to change an array's keys: array_change_key_case(). Unfortunately this can only transform keys to upper- or lower-case.

Because this function doesn't allow developers to specify their own custom logic to transform keys to something else, one must typically resort to using a foreach loop to build a new array:

$newArray = [];
foreach ($oldArray as $key => $value) {
    $newKey = 'someValue'; // Whatever custom logic is needed
    $newArray[$newKey] = $value;

This approach requires 4 lines of code. Furthermore, this logic cannot be wrapped as a parameter to some other function.

That latter issue can solved by composing several existing functions like so:

$newArray = array_combine(
        function ($key, $value) {
            return 'someValue'; // Whatever custom logic is needed

While this does work, it has some major drawbacks:

  • The code is convoluted - its purpose is not immediately obvious.
  • Requires at least 3 separate function calls, each of which returns a new array.
  • The input array is referenced 3 times.
    • If you want to re-key an iterator's results, you cannot do so inline - a temporary variable would be needed to convert the iterator to array first.


This RFC proposes the creation of a new core function array_change_keys():

$newArray = array_change_keys($originalArray, function ($key, $value) {
    return 'someValue'; // Whatever custom logic is needed

Function definition:

array array_change_keys(array $originalArray, callable $callback)

This function takes two arguments:

  1. An array to re-key.
  2. A callable which returns a new key for each array element.

(This parameter order is consistent with all other array functions except for array_map(), which is a special case due to its variadic nature.)

A new array will be returned from this function, containing the same values in the same order but with potentially different keys. Some values may not be included if an invalid or duplicate key is returned by the callback. This behavior is identical to array_flip() and is documented in the “Callback” subsections further below.


Two parameters will be passed to the callable for each element in the array:

  1. The element's original key (string or int)
  2. The element's original value (mixed)

The callable must return a string or int to be used as the new key.

Returning Invalid Types

The callable must return a valid key. Returning any type besides string or int will result in the following warning:

Warning: array_change_keys(): New key should be either a string or an integer

Additionally, the current array item will not be added to the resulting array. PHP will still attempt to process all subsequent elements.

This matches the behavior of calling array_flip on an array containing types other than string or int.

Returning Duplicate Keys

If the callable returns the same key for multiple values, the last occurrence “wins” and all prior values will be lost. For example:

var_dump(array_change_keys([1, 2, 3], function(){ return 'foo'; }));
// array(1) {
//   ["foo"]=>
//   int(3)
// }

This behavior also matches array_flip().

Function Name

PHP already has an array_change_key_case() function, so sharing a common root name (array_change_key) seems like a logical choice.

Other functions which deal with multiple keys (like array_keys and array_fill_keys) are pluralized, so we're using that same convention here.


This section will be updated with any additional pros/cons that arise during the discussion period.


Provides Common General-Purpose Functionality

PHP already has an array_change_key_case function, which is an incredibly specific implementation that isn't useful in the majority of cases where an array needs to be re-keyed. By providing a general-purpose function for a common problem we prevent the need for other array_change_key_* variants in the future.


Needing to re-key array is a common task for some PHP developers, especially those needing their array to work like a dictionary.

Cleaner Code

Using this function makes it immediately obvious to other developers that an array is being re-keyed.

Matches Existing Behavior

The “edge cases” mentioned above (returning invalid types or duplicate keys) matches existing behavior in PHP that developers already understand and expect. No new edge cases or quirks are being introduced with this RFC.

Faster Execution Than array_combine()

Re-keying an array with array_change_keys() is faster than the array_combine approach:

(Benchmarks generated with https://github.com/Ocramius/array_change_keys-benchmark)

Works With Functional Code

This function can be nested inside of other method calls for function composition. The same is not possible for the foreach approach (without requiring the creation of a separate method to encapsulate that functionality).


Slower Than foreach

As noted in the benchmarks above, the foreach loop approach is faster than array_change_keys in most (but not all) cases.

Does Not Support Traversable

Like the other array_ functions, this one also doesn't support iterators, which may be seen as a step backwards.

Easily Implemented In User Land

This function can be implemented in user land using one of the alternative approaches shown above. There's a general feeling among some developers that “what can be implemented in userland shouldn't be in core”.

Backward Incompatible Changes


Proposed PHP Version(s)

Next PHP 7.x release

RFC Impact


This RFC should not impact the SAPI's.

To Existing Extensions

No existing extensions are affected.

To Opcache


Open Issues


Future Scope

Other array_ functions do not support certain features like using __toString() for keys or supporting Traversable objects like iterators, so support for them is not being proposed here either. If such functionality is desired, a separate RFC could be created to add this functionality to all similar functions.


Because this is not a language change, a 50%+1 vote will be required to add this new function.

Patches and Tests

A proposed implementation is provided with this RFC: https://github.com/php/php-src/pull/1925

Several tests are also included.


Links to external references, discussions or RFCs

Mailing list discussion: https://marc.info/?l=php-internals&m=146452769326964&w=2

Reddit discussion: https://www.reddit.com/r/PHP/comments/4ll1hg/rfc_array_change_keys/

rfc/array_change_keys.txt · Last modified: 2018/11/27 23:34 by colinodell