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
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    with Xdebug v3.5.0, Copyright (c) 2002-2025, by Derick Rethans

Operating System

No response

まず、mb_detect_encoding() を使用した header_register_callback の例を再現し、ドキュメントに記載された期待される空の出力と比較します。ディスカッションとリンクされている pull request #20824 を読み、その後、モジュールリクエストのシャットダウン後に callback がユーザーコードを実行しなくなったこと、および再現時に fatal error が出力されなくなったことを確認します。