Next index for array returned from `array_keys()` is wrong

[nox-4O4]

Description

The following code:

<?php

$a = array(123 => 123);
unset($a[123]);

$b = array_keys($a);
$b[] = 42;

var_dump($b);

Resulted in this output:

array(1) {
  [124]=>
  int(42)
}

But I expected this output instead:

array(1) {
  [0]=>
  int(42)
}

The behavior changed during a patch version update: https://3v4l.org/KR2JQ#veol

This only happens when array_keys() is provided with an empty array that previously contained some elements.

PHP Version

PHP 8.4.25 (cli) (built: Aug 28 2026 07:36:03) (NTS)
Copyright (c) The PHP Group
Built by Debian
Zend Engine v4.4.25, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.25, Copyright (c), by Zend Technologies

Operating System

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