As far as i'm aware it's not necessary to have published work prior to submission, and it seems to depend on the type of research you undertake how much potential there is for publication during intermediary stages.

My research is in modular structure (due to the type of work I do) which means I have been able to submit five conference papers, plus one journal submission. I know of others who have had to wait until their research is complete before thinking about suitable targets though.

I think it's important however to realise however that publishing is not the main objective. Getting your PhD is!