Heh, I’ve written a LOT of TCP and UDP (and SCTP) protocols over the past 25 years, many in erlang itself, so I like to help when I can. ^.^
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Interesting post, how would you write the PHP part in Elixir. I actually need it 
:ok = :gen_tcp.send(socket, payload)
I understood I need to add the size of my payload in the 32 firsts bits of what I send but how would I do that ?
As I recall if you set {packet, 4} in the connect then when you :gen_tcp.send/2 it then it should add it automatically. If however you want to do it manually (like you did not specify a packet option at all then you could do:
payload = :erlang.iolist_to_binary(payload) # Do this if it is not already a binary
:ok = :gen_tcp.send(socket, [<<byte_size(payload)::integer-size(32)>>, payload])
Most Erlang IO functions take IO lists, so that is that above. 
You can also bake it into the payload binary itself, but this is slower as it has to copy the entire payload (assuming it is a binary and not an IOlist too):
:ok = :gen_tcp.send(socket, <<byte_size(payload)::integer-size(32), payload::binary>>)
Thank you very much !! Indeed I used packet: :line at the beginning and forgot to change it in the connect options 
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