mail() throws TypeError after iterating over $additional_headers array by reference #10990

Description

The following code:

<?php
$headers = ['From' => '[email protected]'];
foreach ($headers as $n => &$v) ;
mail('[email protected]', 'Test', 'Test', $headers);

... resulted in this output in PHP 8.x:

PHP Fatal error:  Uncaught TypeError: Header "From" must be of type array|string, string given in /home/richard/test.php:4
Stack trace:
#0 /home/richard/test.php(4): mail()
#1 {main}
  thrown in /home/richard/test.php on line 4

I can't see how this can be anything other than a bug.

(The message is worded slightly different in 7.x, but clearly the same issue. It also proceeds to send the message but with no additional headers at all. I appreciated PHP 7.x is unsupported – I include this for information only. In PHP 8.x, the message is not sent at all.)

Comment out the iteration and it works (i.e. produces no output). It also works if you replace the iteration with one by value, i.e.

foreach ($headers as $n => $v) ;

Adding print_r($headers) before and after the iteration show, as expected, that the array has not changed.

Attempted workarounds like calling reset($headers) (in case it's some weirdness with internal array pointers), doing $v = (string)$v in the loop, or copying the array, either directly or indirectly with $headers = array_merge([], $headers); do not help. Workarounds that entirely avoid iteration by reference, e.g. by building up a second array, do work.

PHP Version

PHP 8.1.13, 8.0.26, 7.4.33 and 7.3.31

Operating System

Debian 11