Description:
------------
The documentation states (c.f. https://www.php.net/manual/en/ini.core.php#ini.default-charset):

"In PHP 5.6 onwards, "UTF-8" is the default value and [...] The value of default_charset
will also be used to set the default character set for [...] and for mbstring functions
if the mbstring.http_input mbstring.http_output mbstring.internal_encoding
configuration option is unset."

As such, I'd expect to be able to set default_charset to iso-8859-1 and mbstring to pick that same setting for its internal encoding (if the mentioned directives are unset, that is).

In the test script below the output of mb_strlen should be "2" in both cases, as iso-8859-1 is a single byte encoding. This is not the case, instead it seems that the two bytes are recognized as the utf-8 character "ö".

If mb_internal_encoding is set explicitly, the mb-functions work as expected.

But it should in fact not be needed to use mb_internal_encoding() nor the ini setting for it, because default_charset should be used as default.





Test script:
---------------
<?php

ini_set('default_charset', 'iso-8859-1');
var_dump(ini_get("mbstring.internal_encoding"));
var_dump(ini_get("mbstring.http_input"));
var_dump(ini_get("mbstring.http_output"));
var_dump(mb_internal_encoding());
var_dump(mb_strlen( "\xc3\xb6" ));
var_dump(mb_strlen( "\xc3\xb6", '8bit' ));

mb_internal_encoding('iso-8859-1');
var_dump(mb_internal_encoding());
var_dump(mb_strlen( "\xc3\xb6" ));
var_dump(mb_strlen( "\xc3\xb6", '8bit' ));

Expected result:
----------------
string(0) ""
string(0) ""
string(0) ""
string(5) "UTF-8"
int(1)
int(2)
string(10) "ISO-8859-1"
int(2)
int(2)

Actual result:
--------------
string(0) ""
string(0) ""
string(0) ""
string(5) "UTF-8"
int(2)
int(2)
string(10) "ISO-8859-1"
int(2)
int(2)