User code can run after module request shutdown via the output layer

Description

The following code:

<?php
header_register_callback( fn() => mb_detect_encoding('foo') );

Resulted in this output:

Fatal error: Uncaught ValueError: mb_detect_encoding(): Argument #2 ($encodings) must specify at least one encoding in /in/0EDPU:2
Stack trace:
#0 /in/0EDPU(2): mb_detect_encoding('foo')
#1 [internal function]: {closure:/in/0EDPU:2}()
#2 {main}
  thrown in /in/0EDPU on line 2

But I expected this output instead:

The error seems to be present since PHP 8.0.0
https://3v4l.org/0EDPU

The error also occurs when mb_convert_encoding() is called instead of mb_detect_encoding().

Workaround

Setting mbstring.detect_order explicitly in php.ini prevents the error:

; php.ini
mbstring.detect_order = "ASCII,UTF-8";

Setting the order with mb_detect_order() inside the callback also prevents the issue:

header_register_callback( function() {
    mb_detect_order(['ASCII', 'UTF-8']);
    mb_detect_encoding('foo');
} );

Setting the order outside of the callback or setting it with ini_set() did not prevent the error.

PHP Version

PHP 8.4.15 (cli) (built: Dec  8 2025 22:51:21) (NTS)
Copyright (c) The PHP Group
Built by https://github.com/docker-library/php
Zend Engine v4.4.15, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.15, Copyright (c), by Zend Technologies
    with Xdebug v3.5.0, Copyright (c) 2002-2025, by Derick Rethans

Operating System

No response

Start by reproducing the header_register_callback example with mb_detect_encoding() and compare it with the documented expected empty output. Read the discussion and linked pull request #20824, then verify that the callback no longer runs user code after module request shutdown and that the reproduction no longer emits the fatal error.