Arrays starting with a negative index View as Markdown

Introduction

On the documentation for array_fill the start_index description mentions: “If start_index is negative, the first index of the returned array will be start_index and the following indices will start from zero.” This is consistent everywhere when an explicit negative key is used and implicit keys are used afterwards.

Proposal

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 also 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.

This proposal targets 8.0. However, to ease the transition and find places where this may become an issue, there is a secondary vote for emitting a deprecation notice in cases where the behavior will change.

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)
}

NOTE: If accepted, during the deprecation phase the following E_DEPRECATED notice would be emitted in cases where the behavior will change:

Deprecated: In the next major version of PHP the implicit keys of this array will start from -1 instead of 0 in ...

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

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

Backward Incompatible Changes

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; // Current: Key is -2, RFC: Key is -2
$a[] = true; // Current: Key is 0, RFC: Key is -1
$a[] = true; // Current: Key is 1, RFC: Key is 0
 
if ($a[1] === true) {
    echo 'Accessing key 1 explicitly';
}

Will no longer output Accessing key 1 explicitly.

NOTE: If accepted, during the deprecation phase the E_DEPRECATED notice mentioned in the previous section would be emitted.

Proposed PHP Version(s)

PHP 8.0 (Deprecation notice for 7.3)

Unaffected PHP Functionality

Arrays with explicit keys, string keys, or an initial numeric index >= -1 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.

Voting Choices

Both votes require a 2/3 majority.

Vote: Arrays starting with a negative index for 8.0 — Yes: 17 · No: 2 (closed)

Vote: Deprecation notice for 7.3 — Yes: 8 · No: 14 (closed)

The previous vote targeting 7.2 was closed with 14 for and 16 against

Patches and Tests

This RFC is implemented by the following patches:

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

References

Version 0.3:

Discussion on the mailing list: https://externals.io/message/98302

First proposal Voting period discussion: https://externals.io/message/99412

Second proposal Voting period discussion: https://externals.io/message/99511

Version 0.4:

Discussion on the mailing list: https://externals.io/message/101861