ß is a tricky case: IDNA 2003 had the character be converted to "ss" while IDNA 2008 fixes it. To resolve the confusion UTS#46 was created that basically says to keep using the old behavior. That means if you try your domain in a browser today you'll probably see it silently convert to "ss"; a few WHOIS services I tried also interpreted my request as one for the "ss" version, and apparently some registrars require clients to register both the ß- and ss-versions of domain names.

Here's one of many people talking about it:
https://blogs.msdn.microsoft.com/shawnste/2010/12/08/idna-is-it-fussball-or-fuball/

So basically the default behavior for just about everybody should be to accept "ss" as the default conversion.

The good news is that PHP has both options available. Check the documentation, then use the IDNA_NONTRANSITIONAL_TO_ASCII $options and the INTL_IDNA_VARIANT_UTS46 $variant.
https://3v4l.org/dGENe