The docs I find about is_resource() on the net are not really chatty about this function but I’d guess that is_resource() returns true if the socket you opened is bound to an external resource that needs cleanup after use.
With a strongly typed language like Go, this check is not necessary. If you open a socket, then your variable is a socket.
(Same for all the other is_* functions I find here – is_bool(),is_array(), etc.)