PHP RFC: Arrays starting with a negative index

• Version: 0.1
• Date: 2017-04-20
• Author: Pedro Magalhães, mail at pmmaga dot net
• Status: Draft
• First Published at: http://wiki.php.net/rfc/negative_array_index

When a developer adds an element to an array with a numeric key and later adds a new element to that array without specifying a key, the key of the new element depends on the sign of the first key. This RFC proposes to make this consistent by always using the value of the first key + 1 for the new element.

In other words, any array that has a number n as it's first numeric key will have for it's next implicit key either n+1 if n >= 0 or 0 if n < 0. This RFC proposes to make this consistent by always using n+1 regardless of the sign of n.

As mentioned on the relevant part of the arrays documentation:

if no key is specified, the maximum of the existing integer indices is taken, and the new key will be that maximum value plus 1 (but at least 0).

This RFC proposes to eliminate the parenthesis part of that sentence. Which is an exception to a rule, that people have to learn one way or the other.

All the variables in this code:

<?php
 
$a = array_fill(-2, 3, true);
$b = [-2 => true, true, true];
$c = ["string" => true, -2 => true, true, true];
unset($c["string"]);
$d[-2] = true;
$d[] = true;
$d[] = true;

Result in the array:

array(3) {
  [-2]=>
  bool(true)
  [0]=>
  bool(true)
  [1]=>
  bool(true)
}

With the implementation of this RFC, they will all result in:

array(3) {
  [-2]=>
  bool(true)
  [-1]=>
  bool(true)
  [0]=>
  bool(true)
}

Code that relies on the current behavior to access array elements with an explicit key after that array has been started with a negative index and used implicit keys afterwards will change it's behavior. Example:

<?php
 
$a[-2] = true;
$a[] = true;
$a[] = true;
 
if ($a[1] === true) {
    echo 'Save the world.';
}

Will no longer save the world.

PHP 7.2

Arrays with explicit keys, string keys, or an initial numeric index >= 0 are not affected. This also means that arrays that never use explicit keys will still start with 0.

Furthermore, iterating over arrays without explicit keys (ie. foreach) is not affected.

This RFC requires a 2/3 majority.

This RFC is implemented by the following patches:

1. Implementation: https://github.com/php/php-src/pull/2383
2. Documentation: WIP

Discussion on the mailing list:https://externals.io/thread/712