Description:
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Lots of bugs have beed opened on this subject, lots of bugs have beed closed / resolved / etc : https://bugs.php.net/search.php?search_for=turkish&boolean=0&limit=30&order_by=&direction=DESC&cmd=display&status=All&bug_type=All&project=All&php_os=&phpver=&cve_id=&assign=&author_email=&bug_age=0&bug_updated=0

Those bugs typically address iso8859-9, the latin-5 character encoding used in Turkey, this one addresses the UTF8 encoding:

In turkish:
SMALL I capitalized is CAPITAL I WITH DOT ABOVE (and vice-versa)
SMALL DOTLESS I capitalized CAPITAL I (and vice-versa)

In PHP I got (see test script)
Expects CAPITAL I WITH DOT ABOVE: iI
Expects SMALL I WITHOUT DOT: Ii

Which is, more or less, wrong for the 4 tests.

Test script:
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<?php

mb_internal_encoding('UTF-8');
setlocale(LC_ALL, 'tr_TR.UTF-8');
echo "Expects CAPITAL I WITH DOT ABOVE: " . strtoupper('i') . mb_strtoupper('i', 'UTF-8') . PHP_EOL;
echo "Expects SMALL I WITHOUT DOT: " . strtolower('I') . mb_strtolower('I', 'UTF-8') . PHP_EOL;

Expected result:
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Expects CAPITAL I WITH DOT ABOVE: İİ
Expects SMALL I WITHOUT DOT: ıı

Actual result:
--------------
Expects CAPITAL I WITH DOT ABOVE: iI
Expects SMALL I WITHOUT DOT: Ii