Two foreach loops over the same array produces incorrect results when first loop is by ref and second is by val

[jlcfly]

Description

The following code:

<?php
$arr = [1, 2, 3];

echo "first foreach\n";
foreach($arr as &$a) {
    echo "{$a}\n";
}

echo "second foreach\n";
foreach($arr as $a) {
    echo "{$a}\n";
}

Resulted in this output:

first foreach
1
2
3
second foreach
1
2
2

But I expected this output instead:

first foreach
1
2
3
second foreach
1
2
3

Iterating over the same array twice with foreach when the first one is by ref and the second one is by val makes the last two values of the second loop the same. It's a very particular behavior that I happened to stumble across in code today that was creating an odd issue. This is version agnostic. I tested all the way back to 5.6, so this behavior has been around for a while.

If both loops are by ref or both are by val, it behaves correctly. If the first loop is by val and second is by ref, it also behaves correctly. Only when the first is by ref and the second is by val does it misbehave.

PHP Version

PHP 8.3.10

Operating System

No response

[nielsdos]

This is a classic pitfall with how references work.
TLDR: $a reference is still bound to the last element of the array when the second loop starts. This causes $a to eventually become the second to last value of the array. An easy fix is to unset the variable before the second loop.
Full explanation in the red warning box in https://www.php.net/manual/en/control-structures.foreach.php