mb_strrpos() returns the wrong position for a negative offset in a non-UTF-8 encoding

[eyupcanakman]

Description

The following code:

<?php
var_dump(mb_strrpos("\xA9\xA9X", "\xA9", -2, "ISO-8859-1"));
var_dump(strrpos("\xA9\xA9X", "\xA9", -2));

Resulted in this output:

int(0)
int(1)

But I expected this output instead:

int(1)
int(1)

For a single-byte encoding one byte is one character, so mb_strrpos() should agree with strrpos() here. The result is wrong for a negative offset in any non-UTF-8 encoding. An ISO-8859-1 needle byte in 0x80-0xBF gives a position that is too early, and an ASCII needle in UTF-16 also returns the wrong position.

PHP Version

PHP 8.4.25-dev (the same code is in 8.3, 8.5 and master)

Operating System

macOS 27.0

[youkidearitai]

Thanks for report. I confirmed behavior changes from PHP 8.3
https://3v4l.org/MpZOD#veol

@alexdowad What do you think?

[alexdowad]

@youkidearitai I commented in @eyupcanakman's PR.