Description:
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Example on mb_convert_case() does not show the utility of the function, since the sample text has no accents or special characters.

It will be useful a comparison with strtoupper() / strtolower()

I proppose the following better examples:

$str= "Thís is à tëxt with çpécýal châráctèrs";
echo strtoupper($str); //prints a corrupted text in UTF: TH�S IS � T�XT WITH �P�C�AL CH�R�CT�RS
echo mb_convert_case($str, MB_CASE_UPPER, "UTF-8"); // prints a valid upper case conversion in UTF: THÍS IS À TËXT WITH ÇPÉCÝAL CHÂRÁCTÈRS

$str = "mary had a Little lamb and she loved it so";
echo = mb_convert_case($str, MB_CASE_TITLE, "UTF-8"); // prints: Mary Had A Little Lamb And She Loved It So