Description:
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In most languages, the callback of Array's reduce() method receives the current index (as well as $result and $item). In PHP it doesn't. There's plenty of room for a third argument, since it's never used.

The test script assumes that 3rd argument.

Test script:
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// Compare string lengths and remember index of the longest.

$strings = ['foo', 'bar', 'fooo', 'barry', 'barrr'];
$longest_index = array_reduce($strings, function($result, $string, $index) use ($strings) {
  return strlen($string) > strlen($strings[$result]) ? $index : $result;
}, 0);

// Re-key array depending on values.

$array = ['10' => 'a', '20' => 'b', '150' => 'c'];
$rekeyed = array_reduce($array, function($result, $el, $index) {
  $new_index = $el > 100 ? 'x' . $index : 'y' . $index;
  $result[$new_index] = $el;
  return $result;
}, []);


Expected result:
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I expect $index in the callbacks to be present, and:

$longest_index should be 3 (for 'barry'), and

$rekeyed should have indexes 'x10', 'x20' and 'y150'.

Actual result:
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Warnings and nils.