mb_check_encoding returns true for € and ISO -8859-1 instead of false

[jWeck-Software]

Description

The following code:

<?php

$str = '€';
var_dump(mb_check_encoding($str, 'ISO-8859-1'));

Resulted in this output:

true

But I expected this output instead:

false

https://en.wikipedia.org/wiki/ISO/IEC_8859-1#:~:text=by%20this%20standard.-,Euro%20sign,-%5Bedit%5D

The euro sign was first presented to the public on 12 December 1996.[13] Due to this character set being introduced in 1987, it does not include the euro sign. Later character sets similar to ISO/IEC 8859-1 include a euro sign, such as Windows-1252 and ISO/IEC 8859-15.

PHP Version

PHP 8.3.23 (cli|fpm) (built: Jul  3 2025 16:10:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.23, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.23, Copyright (c), by Zend Technologies

Operating System

Kubuntu 22.04, Ubuntu 24.04, Debian 12

[alexandre-daubois]

ISO-8859-1 is handled with byte-level validation, not semantic validation. I guess a custom validation function should be added in mbfilter_singlebyte.c to detect potential UTF sequences?

[youkidearitai]

This is not a bug. Because € is 0xe282ac in UTF-8.
https://3v4l.org/RgNYT#veol

You should try to convert to ISO-8859-1.
https://3v4l.org/QbnEE#veol