  We present mean horizontal branch absolute magnitudes and iron abundances for
a sample of 39 globular clusters. These quantities were calculated in an
unprecedented homogeneous fashion based on Fourier decomposition of ligt curves
of RR Lyrae cluster members. Zero points for the luminosity calibrations are
discussed. Our photometrically derived metallicities and distances compare very
well with spectroscopic determinations of [Fe/H] and accurate distances
obtained using {\sl Gaia} and {\sl Hubble Space Telescope} data. The need to
distinguish between the results for RRab and RRc stars for a correct evaluation
of the $M_V$--[Fe/H] relation is discussed. For RRab stars, the relation is
non-linear, and the horizontal branch structure plays a significant role. For
RRc stars, the relation remains linear and tight, and the slope is very
shallow. Hence, the RRc stars seem better indicators of the parental cluster
distances. Systematic time-series CCD imaging performed over the last 20 years
enabled to discover and classify 330 variables in our sample of globular
clusters.
