Unix Domain: Unix and UDG

unix:// and udg:// (udg:// since PHP 5).

unix:// provides access to a socket stream connection in the Unix domain. udg:// provides an alternate transport to a Unix domain socket using the user datagram protocol.

Unix domain sockets, unlike Internet domain sockets, do not expect a port number. In the case of fsockopen() the portno parameter should be set to 0.