UTF8 and Slim3 not using mb_* functions

I read through http://www.phptherightway.com/#php_and_utf8 and looked at the Slim 3 codebase and discovered that there are a number of occurrences of non mb_* functions, such as in Headers.php

    public function normalizeKey($key)
        $key = strtr(strtolower($key), '_', '-');
        if (strpos($key, 'http-') === 0) {
            $key = substr($key, 5);

        return $key;

Is this important if I want to ensure that my app is 100% uft8 compliant?

I think you’ll find that cases like this are fine as HTTP header key names are ASCII.