I chased the code path since I was looking at PHP network stuff a bit ago, and it (through a ton of indirection) basically just does getsockname then calls inet_ntop on it based on the returned sockaddr's address family. There's not much additional smarts in that code path other than getting the underlying socket fd for the stream. I'm feeling this might be DNS. Probably, it resolves localhost as ::1 and tries making the socket and connecting to that, even if (for some reason) it can't.