Special characters and their output (utf-8)

I struggle with the problem that special characters (Ö, Ü, Ä, ß) are not output correctly. I have check these points:

- <meta charset="utf-8">
- $config['charset'] = 'UTF-8';
- database: utf8_unicode_ci


I have already written a function, that works even with normal output:

PHP Code:

function utf8_ausgabe ($str) {
    
    return mb_convert_encoding($str, 'HTML-ENTITIES', 'utf-8');
} 


... but in form inputs, it doesn't work. Of course, as well as the '&' is converted.

without convert function:

with convert function:

Do you have a solution for this?

Thanks in advance