Why does an empty IP address for fsockopen() on port 80 return a resource

Question

I am using fsockopen() to check for open ports. It seems to be working when an active IP address is passed, but seems to always return a resource when the IP address parameter is blank or NULL when using port 80 or 22. What does this special case mean?

fsockopen("", 80, $err_code, $err_msg, 1) returns a resource

fsockopen("123", 80, $err_code, $err_msg, 1) returns FALSE

I would expect both to return FALSE.