Description:
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mbstring's mb_strtoupper (and partners) do "simple casemapping" instead of the recommended full casemapping.

http://www.unicode.org/versions/Unicode8.0.0/

Section 4.2 "Case", Headline "Case Mapping"
"The single-character mappings in UnicodeData.txt are insufficient for languages such as German. Therefore, only legacy implementations that cannot handle case mappings that increase string lengths should use UnicodeData.txt case mappings alone."

PHP should stop being "legacy" and "insufficient".

For implementation guidelines, see section 5.18 "Case Mappings" and
ftp://ftp.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt


PHP's current implementation: 
php-src/ext/mbstring/ucgendat/ucgendat.c
seems to ignore "SpecialCasing.txt". 
The data structure
 static const unsigned int _uccase_map[]
in php-src/ext/mbstring/unicode_data.h assumes (IMHO) a one-to-one mapping.

BTW:
Perl, Python, Java and others all do full case mapping.

This is *not* a bleeding edge feature. Perl has this since version 5.8.0 (year 2002!!!).


Test script:
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$str="\xc3\x9f"; echo $str." upper => ".mb_strtoupper($str,'UTF-8')."\n";
$str="\xc5\x89"; echo $str." upper => ".mb_strtoupper($str,'UTF-8')."\n";


Expected result:
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ß upper => SS
ʼn upper => ʼN


Actual result:
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ß upper => ß
ʼn upper => ʼn