Description:
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On this page :

http://php.net/manual/fr/function.mb-convert-case.php

The example 1 shows :

<?php
$str = "Marie a un petit agneau et l'aime tr?s fort";
$str = mb_convert_case($str, MB_CASE_UPPER, "UTF-8");
print $str;
// Affiche : MARIE A UN PETIT AGNEAU ET L'AIME TR?S FORT
(...)

However the correct output given by mb_convert_case() is :

MARIE A UN PETIT AGNEAU ET L'AIME TR?S FORT

This example should demonstrate that mb_convert_case() can handle characters like ?/?/?/?/etc, thing that strtoupper() can't do.