This function is very handy when dealing with UDP connections, because it enables you to know who's the client who connected to your socket. Bear in mind that UDP doesn't care about the source of the connection, the packets may be annonymous or even faked. No check is required.
If you want to listen on an UDP socket and answer to the client, read my comment on socket_listen() -> http://www.php.net/manual/en/function.socket-listen.php