Popper's test of quantum mechanics and two-photon "ghost" diffraction

A test on quantum mechanics proposed long ago by Karl Popper is reconsidered with further detail and new insight. An ambiguity in the proposal, which turns out to be essential in order to make the test conclusive, is identified and taken into account for the first time. Its implications for recently performed photon experiments [such as in D. V. Strekalov \textit{et al.}, Phys. Rev.\ Lett. {\bf 74}, 3600 (1995)] are briefly analyzed.