  We report the mean metallicity and absolute magnitude of RR Lyrae stars in a
sample of 37 globular clusters, calculated via the Fourier decomposition of
their light curves and ad hoc semiempirical calibrations, in an unprecedented
homogeneous approach. This enabled a new discussion of the metallicity
dependence of the horizontal branch (HB) luminosity, as a fundamental distance
indicator. The calibration for the RRab and RRc stars should be treated
separately. For the RRab the dispersion is larger and non-linear. For the RRc
stars the correlation is less steep, very tight and linear. The relevance of
the HB structural parameter L, is highlighted and offer a non-linear
calibration of the form MV ([Fe/H],L). Excellent agreement is found between
values of [Fe/H] and MV from the light curve decomposition with spectroscopic
values and distances obtained via Gaia-DR3 and HST. The variable stars census
in 35 clusters includes 326 stars found by our program.
