Description:
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Hi,

I'm currently working on a group of classes abstracting operations on ASCII and UTF8 strings using either the native string functions or multibytes functions or graphemes cluster functions from intl extension.

It seems the grapheme_strripos() and grapheme_strrpos() functions don't calculate a position right when they receive an accentuated string AND a negative offset. However the position calculus is correct when these functions receive an accentuated string AND a positive offset.

If you look at the attached piece of test script, the statements #5 and #6 are expected to return int(5) but they return int(6) instead. If you try this behavior with mb_strrpos() and mb_strripos() functions instead, the output will be int(5) as expected.

You can find the tested scenario here: https://3v4l.org/UmN6B

Test script:
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// Expecting int(6) [3 times]
var_dump(grapheme_strrpos('dejaaaa', 'a', 2));   // int(6)
var_dump(grapheme_strrpos('déjàààà', 'à', 2));   // int(6)
var_dump(grapheme_strripos('DÉJÀÀÀÀ', 'à', 2));  // int(6)

// Expecting int(5) [3 times]
var_dump(grapheme_strrpos('dejaaaa', 'a', -2));  // int(5)
var_dump(grapheme_strrpos('déjàààà', 'à', -2));  // int(6)
var_dump(grapheme_strripos('DÉJÀÀÀÀ', 'à', -2)); // int(6)

// Expecting int(5) [3 times]
var_dump(mb_strrpos('dejaaaa', 'a', -2));        // int(5)
var_dump(mb_strrpos('déjàààà', 'à', -2));        // int(5)
var_dump(mb_strripos('DÉJÀÀÀÀ', 'à', -2));       // int(5)