idn_to_ascii('faß.de');
will return fass.de
(Due to Germans using the ß symbol as a contraction of ss)
idn_to_ascii('faß.de', IDNA_NONTRANSITIONAL_TO_ASCII);
will return xn--fa-hia.de
(This is the correct behavior too allow faß.de and fass.de to be registered as different domain names.)
idn_to_utf8('xn--fa-hia.de');
will return fass.de
idn_to_utf8('xn--fa-hia.de', IDNA_NONTRANSITIONAL_TO_UNICODE);
will return faß.de
(Again this is the correct behavior.)