Socket Select()

Hi, I'm working on writing a multi-client socket server and have a question about the socket_select() function. I've read through lots of example source on the web using BSD sockets and select(). For some reason none of them ever populate the write fd set. They only populate the read fd set. $numchanged = socket_select($read,$write=NULL,$except=NULL,0); The server examples I've seen usually just test for any incoming data and then respond immediately with a socket_write, they never bother using the write fd set to see if the socket can be written to. Can someone explain why, and maybe point me to some examples where it would be useful to use the write fd set? thanks alot, will