Getting fd from stream

Hello internals@, I have an (exotic) case where I need to be able to get the filedescriptor from a previously opened stream (via stream_socket_pair) to hand over to the child process, which needs to reexecute. I saw that the php:// wrapper supports php://fd/X syntax, so I reckon there should be a way to extract the information as well from a stream as well. Unfortunately I don't seem to be able to find it. Can someone point me in the right direction? Sincerely, Martijn van Duren