Description:
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I saw your example code in http://php.net/manual/en/migration73.new-features.php#migration73.new-features.mbstring, but unfortunately the result in PHP 7.3 is not really true.

At 06/29/2017, germany adopted capital ß (ẞ) into their orthography, so uppercase s should result in ẞ instead of SS.



It's also noted in wikipedia:
- https://en.wikipedia.org/wiki/%C3%9F

"In 2017, the Council for German Orthography ultimately adopted capital ß (ẞ) into German orthography, ending a long orthographic debate."

Test script:
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<?php

mb_strtoupper("Straße");

?>

Expected result:
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STRAẞE

Actual result:
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STRASSE