[php-src] Issue #7871: `mb_detect_encoding()` detects UTF-8 emoji byte sequence as ISO-8859-1 since PHP 8.1

Issue: https://github.com/php/php-src/issues/7871 Author: filecage ### Description I have a piece of code that tries to normalise the encoding of incoming strings using mb_detect_encoding(). While upgrading to PHP 8.1, I've noticed that a test which ensures that an urlencoded UTF-8 sequence (party hat emoji) now fails. It all comes down to a behaviour change of mb_detect_encoding() when passing UTF-8 and ISO-8859-1 (in which ever order) as $encodings in PHP 8.1. I know that the way this method works (or the way that determining the enconding of a string works at all) can not be 100% realiable, so I'd also agree on not classifying this as a bug. However, it is an undocumented behaviour change introduced in 8.1 that might break existing code as it did with mine. I assume that this change has been introduced with 28b346bc. Example (see https://3v4l.org/RgdfE): ```php <?php echo mb_detect_encoding('🥳', ['UTF-8', 'ISO-8859-1']); ``` Resulted in this output: ``` ISO-8859-1 ``` But I expected this output instead: ``` UTF-8 ``` ### PHP Version 8.1.1 ### Operating System _No response_