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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Operating System

No response

Bắt đầu bằng cách tái hiện ví dụ header_register_callback với mb_detect_encoding() và so sánh với đầu ra rỗng dự kiến được ghi trong tài liệu. Đọc phần thảo luận và pull request #20824 được liên kết, sau đó xác minh rằng callback không còn chạy mã người dùng sau khi module request shutdown và việc tái hiện không còn phát ra lỗi fatal.