It looks like that mysterious flags are just the recv(2) flags passed to your OS syscall and nothing more...
ext/sockets/sockets.c:PHP_FUNCTION(socket_recv)
...
if ((retval = recv(php_sock->bsd_socket, recv_buf, len, flags)) < 1) {
efree(recv_buf);
...
for linux you can type `man 2 recv' and you will see complete description of thouse flags.
Sergey S. Kosrtyliov <rathamahata@rathamahata.net>
http://www.rathamahata.net/