Receive UDP Broadcast

Hi!

how can i receive an UDP broadcast to 255.255.255.255 on an dedicated 
interface?

this is my first try, but no luck, how can i enable the broadcast 
feature for my socket?

             $address = 'udp://10.88.0.255:20000';
#            $address = 'udp://10.88.0.20:20000'; // not working
#            $address = 'udp://255.255.255.255:20000'; //also not 
working, so i missing the interface binding to?!

             $context = stream_context_create([
                 'socket' => [
                     'so_broadcast' => 1,
                 ],
             ]);
             $socket = @\stream_socket_server($address, $errno, $errstr, 
\STREAM_SERVER_BIND, $context);

my code is working fine for any unicast packet, but broadcast isnt 
reache this socket. the excepted broadcast-packets can i see with 
tcpdump on my server-intrerface.

any hints for my problem?

regards, volker.