We present a self-consistent theory, as well as analytical and numerical results, for the three-photon-onephoton phase control of resonantly enhanced photoionization in an optically dense medium of xenon gas. We show that for an optically thick medium, the standard phase-control technique has a rather limited application since, independently of the initial relative phase between the two fields, over a very short scaled distance of propagation the medium tends to settle such a relative phase

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... exactly cancels the atomic excitation. Rather unusual results for an optically thin layer of atoms are also found.