How long ago was your interview? It wouldn't hurt to enquire informally, to find out if they would consider your application. However if your interview was quite recent (within a couple of months), it is most likely that none of the people who applied the first time (including you) were quite who they were looking for to fill the position, so they may have advertised again looking to get some different candidates. On the other hand, it might be for a slightly different phd position, related to the first, or maybe the peron they first gave it to has pulled out. Hard to know, but as I say, it doesn't hurt to ask. Good luck.