PHP: rfc:array_key_first_last

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====== PHP RFC: array_key_first(), array_key_last() and array_value_first(), array_value_last() ======
  * Version: 1.1
  * Date: 2018-06-11
  * Author: Enno Woortmann<enno.woortmann@web.de> 
  * Status: Accepted functions array_key_first() and array_key_last() implemented in PHP 7.3
  * First Published at: http://wiki.php.net/rfc/array_key_first_last

===== Introduction =====
As arrays are a powerful data structure it's handy in some cases to get the first or the last key/value of an array without a workaround. To accomplish this task this RFC adds four functions to the core:
  * $key = array_key_first($array);
  * $key = array_key_last($array);
  * $value = array_value_first($array);
  * $value = array_value_last($array);

===== Proposal =====
The current functions of the PHP core only allow to retrieve the first/last key/value of an array by changing the internal state of the array when using the functions reset(), end() and key(). Other implementation approaches are either inperformant (eg. usage of array_keys($array)[0] to gather the first key, which additionally may lead to errors if $array is an empty array) or provide a construct which increases the cognitive complexity of the implementation (eg. usage of loops).

To avoid changes of the internal array representation in order to gather a single information from the array and to increase the comprehensibility of the userland code this RFC proposes new functions for this task. Two functions for the handling of array keys and two functions for the handling of array values:

To gather the **key** of the **first** element of an array the method **array_key_first($array)** will be added.

To gather the **key** of the **last** element of an array the method **array_key_last($array)** will be added.

The usage of the methods to handle the keys will look like:

<code php>
// usage of an associative array
$array = ['a' => 1, 'b' => 2, 'c' => 3];

$firstKey = array_key_first($array);
$lastKey = array_key_last($array);

assert($firstKey === 'a');
assert($lastKey === 'c');

// usage of a numeric array
$array = [1 => 'a', 2 => 'b', 3 => 'c'];

$firstKey = array_key_first($array);
$lastKey = array_key_last($array);

assert($firstKey === 1);
assert($lastKey === 3);

// usage of an empty array
$array = [];

$firstKey = array_key_first($array);
$lastKey = array_key_last($array);

assert($firstKey === null);
assert($lastKey === null);
</code>

To gather the **value** of the **first** element of an array the method **array_value_first($array)** will be added.

To gather the **value** of the **last** element of an array the method **array_value_last($array)** will be added.

The usage of the methods to handle the values will look like:

<code php>
// usage of an associative array
$array = ['a' => 1, 'b' => 2, 'c' => 3];

$firstValue = array_value_first($array);
$lastValue = array_value_last($array);

assert($firstValue === 1);
assert($lastValue === 3);

// usage of a numeric array
$array = [1 => 'a', 2 => 'b', 3 => 'c'];

$firstValue = array_value_first($array);
$lastValue = array_value_last($array);

assert($firstValue === 'a');
assert($lastValue === 'c');

// usage of an empty array
$array = [];

$firstValue = array_value_first($array);
$lastValue = array_value_last($array);

assert($firstValue === null);
assert($lastValue === null);
</code>

All four functions either return the requested key/value or null if an empty array is provided.
If a non array parameter is given, a warning will be triggered and null will be returned.

===== Backward Incompatible Changes =====
None.

===== Proposed PHP Version(s) =====
next PHP 7.x

===== RFC Impact =====
As this RFC adds four new functions there is a possible impact on existing userland extensions where developers created helper functions with an identical name. As the functions proposed provide a meaningful name custom functions should implement an identical functionality and thus be replaceable.

There is no impact to SAPIs or the opcache.

===== Vote =====
This is not a language change so a simple yes/no vote with 50%+1 majority is required.

Voting starts on 2018-07-09 13:30 UTC and closes on 2018-07-16 23:00 UTC.

<doodle title="Add array_key_first() and array_key_last()?" auth="wol-soft" voteType="single" closed="true">
   * Yes
   * No
</doodle>

----

<doodle title="Add array_value_first() and array_value_last()?" auth="wol-soft" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Patches and Tests =====
The GitHub pull request which provides the functionality and the related tests is located at https://github.com/php/php-src/pull/3256

===== Implementation =====

The implementation for array_key_first() and array_key_last() was merged for PHP 7.3 with the commit http://git.php.net/?p=php-src.git;a=commit;h=50516a6

The documentation for the added functions is loated at:

  * http://php.net/manual/en/function.array-key-first.php
  * http://php.net/manual/en/function.array-key-last.php

===== History =====
==== Version 1.1 ====
  * Extended the scope of this RFC to also cover functions for the handling of values to provide a complete function set for working with outer array elements.
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