[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 Comment Author: alexdowad Hmm. One point to start: Legacy versions of PHP return UTF-8, not because they are smart enough to tell that the string is actually UTF-8 text, but because you put UTF-8 first in the list of candidate encodings. If you put ISO-8859-1 first, then all versions return ISO-8859-1. The string is actually valid under both encodings; if interpreted as ISO-8859-1, it's an accented "o", followed by an accented "Y", followed by a Yen sign, followed by a superscript digit 3. A funny combination, but not invalid. The current detection code treats all codepoints higher than U+FFFF as "rare", and penalizes candidate encodings under which the string contains such codepionts. That is what is making it prefer ISO-8859-1 over UTF-8. @nikic Do you think I need to teach it that some emoji over U+FFFF are not "rare"? Another point... no matter what we do, encoding detection will always be very, very unreliable for *short* strings. If you want to auto-detect text encoding with any level of reliability, you really need to give it more input text to work with. Even so, let us think a bit about this issue of emoji. It is a fact that some emoji are used by a lot of people, and they do occupy ranges of Unicode codepoints over U+FFFF.