Look into stream based multicast sockets

[clue]

Initial research suggests that both fsockopen() and stream_socket_client() both issue a connect() call, hence they won't be able to receive any incoming unicast messages in response to outgoing multicast messages.

Also, the stream based API does not expose the required multicast socket options and constants.

Perhaps (ab?)using stream_socket_server() with the appropriate flags and exporting its underlying socket resource via socket_import_stream() might be an option?

Another (somewhat more experimental) option might be to export a stream based socket from an underlying socket resource, see also: 1 2 3