  We studied five XMM-Newton observations of the neutron-star binary 4U
1728$-$34 covering the hard, intermediate and soft spectral states. By jointly
fitting the spectra with several reflection models, we obtained an inclination
angle of 25$-$53$\deg$ and an iron abundance up to 10 times the solar. From the
fits with reflection models, we found that the fluxes of the reflection and the
Comptonised components vary inconsistently; since the latter is assumed to be
the illuminating source, this result possibly indicates the contribution of the
neutron star surface/boundary layer to the disc reflection. As the source
evolved from the relatively soft to the intermediate state, the disc inner
radius decreased, opposite to the prediction of the standard accretion disc
model. We also explore the possible reasons why the supersolar iron abundance
is required by the data and found that this high value is probably caused by
the absence of the hard photons in the XMM-Newton data.
