WHBM
From what I read, this particular PW4000-112 series engine (PW4077?) is the only hollow-bladed version. I don't know about the GE90 engine (except that it has part composite blades in the larger variants at least), but RR Trents nearly all have hollow titanium blades. These are made of three pieces which are placed in a mould, heated to very high temperature and are then internally inflated with high pressure gas to form them to the shape of the mould. The third piece between the two halves of the outer blade faces is corrugated and it stretches into shape with the vertices of the corrugations melting into or fusing with the outer pieces effectively forming a one-piece finished structure.