Default mbstring detect order does not work in header_register_callback()

[nschilling]

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

[youkidearitai]

Seems correct behavior because if mb_detect_encoding of 2nd parameter($encodings) is null, it respect mbstring.detect_order and it default is null.
So it needs set character encoding in $encodings or mbstring.detect_order.

However, I don't know how about works header_register_callback.

[nschilling]

The error does not occur when mb_detect_encoding() is called outside of the callback. So it behaves differently and I don't expect that.😄

mb_detect_order() also returns different values when called inside and outside the callback.

<?php
var_dump(mb_detect_order());

outputs

array(2) {
  [0]=>
  string(5) "ASCII"
  [1]=>
  string(5) "UTF-8"
}

https://3v4l.org/c3WoM

<?php
header_register_callback(function(){
    var_dump(mb_detect_order());
});

outputs

array(0) {
}

https://3v4l.org/HGEi7

[ndossche]

I can explain why this happens.
The callback is executed from within php_output_deactivate, but the mbstring RSHUTDOWN hook was called prior to that via zend_deactivate_modules. The mbstring RSHUTDOWN will clear the list. This reminds me of another issue where user code runs too late (#20656).

You can already see that this is not unique to mbstring, and is purely a shutdown ordering problem, with the following code that also breaks:

<?php
putenv('foo=baz');
header_register_callback(function(){
    putenv('foo=bar');
});

Conceptually, this feels right but would need to double check. It's also a behaviour change (e.g. if RSHUTDOWN would produce output):

diff --git a/main/main.c b/main/main.c
index f190eab3d09..7c45782807f 100644
--- a/main/main.c
+++ b/main/main.c
@@ -1997,16 +1997,16 @@ void php_request_shutdown(void *dummy)
 		zend_unset_timeout();
 	} zend_end_try();
 
-	/* 5. Call all extensions RSHUTDOWN functions */
-	if (PG(modules_activated)) {
-		zend_deactivate_modules();
-	}
-
-	/* 6. Shutdown output layer (send the set HTTP headers, cleanup output handlers, etc.) */
+	/* 5. Shutdown output layer (send the set HTTP headers, cleanup output handlers, etc.) */
 	zend_try {
 		php_output_deactivate();
 	} zend_end_try();
 
+	/* 6. Call all extensions RSHUTDOWN functions */
+	if (PG(modules_activated)) {
+		zend_deactivate_modules();
+	}
+
 	/* 7. Free shutdown functions */
 	if (PG(modules_activated)) {
 		php_free_shutdown_functions();