Strange and confusing behaviour of foreach with iteration by reference

[phront]

Description

The following code:

<?php

$aaa = [[1,2,3,4]];
$i = 5;
foreach ($aaa as &$aa) {
	foreach ($aa as &$a) {
		$a = $i;
		$i--;
	}
}

var_dump ($aaa);
foreach ($aaa as &$aa) {
	foreach ($aa as $a) {
	}
}
var_dump ($aaa);

Resulted in this output:

array(1) {
 [0]=>
 &array(4) {
 [0]=>
 int(5)
 [1]=>
 int(4)
 [2]=>
 int(3)
 [3]=>
 &int(2)
 }
}
array(1) {
 [0]=>
 &array(4) {
 [0]=>
 int(5)
 [1]=>
 int(4)
 [2]=>
 int(3)
 [3]=>
 &int(3)
 }
}

But I expected at least this output instead:

array(1) {
 [0]=>
 &array(4) {
 [0]=>
 int(5)
 [1]=>
 int(4)
 [2]=>
 int(3)
 [3]=>
 int(2)
 }
}
array(1) {
 [0]=>
 &array(4) {
 [0]=>
 int(5)
 [1]=>
 int(4)
 [2]=>
 int(3)
 [3]=>
 int(2)
 }
}

Played the script with different versions on
https://onlinephp.io?s=s7EvyCjg5eLlUklMTFSwVYiONtQx0jHWMYmNtQYKZgKFTIGMtPyi1MTkDAUNsLLEYgU1IENToZqXixNZCioDkeBUARmokmkNZmfq6oIYtbxctSDryhKL4lNKcwsgJmqSZgXMBohhmEaBoIKCAogCAA%2C%2C&v=8.4.10

PHP Version

PHP Version => 8.4.10
Zend Engine v4.4.10, Copyright (c) Zend Technologies

Build Date => Jul 13 2025 00:21:23
Build System => Linux 4.14.290-217.505.amzn2.x86_64 #1 SMP Wed Aug 10 09:52:16 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Operating System

Linux 5.10.214-202.855.amzn2.x86_64 #1 SMP Tue Apr 9 06:57:12 UTC 2024 x86_64

[iluuu1994]

Common pitfall. https://3v4l.org/qRiSf You need to unset $a after the foreach. This is not a bug, though a footgun that cannot easily be changed. You'll find various reports in the bug tracker.