  We study spin models within the mean field approximation to elucidate the
topology of the phase diagrams of systems modeling the liquid-vapor transition
and the separation of He$^3$--He$^4$ mixtures in periodic porous media. These
topologies are found to be identical to those of the corresponding random field
and random anisotropy spin systems with a bimodal distribution of the
randomness. Our results suggest that the presence of walls (periodic or
otherwise) are a key factor determining the nature of the phase diagram in
porous media.
