  Mapping star-formation rates (SFR) within galaxies is key to unveiling their
assembly and evolution. Calibrations exist for computing SFR from a combination
of ultraviolet and infrared bands for galaxies as integrated systems, but their
applicability to sub-galactic (kpc) scales remains largely untested. Here we
use integral field spectroscopy of 19 nearby ($D <$ 20 Mpc) galaxies obtained
by PHANGS-MUSE to derive accurate Balmer decrements (H$\alpha$/H$\beta$) and
attenuation-corrected H$\alpha$ maps. We combine this information with
mid-infrared maps from WISE at 22 $\rm \mu m$, and ultraviolet maps from GALEX
in the far-UV band, to derive SFR surface densities in nearby galaxies on
resolved (kpc) scales. Using the H$\alpha$ attenuation-corrected SFR as a
reference, we find that hybrid recipes from the literature overestimate the SFR
in regions of low SFR surface density, low specific star-formation rate (sSFR),
low attenuation and old stellar ages. We attribute these trends to heating of
the dust by old stellar populations (IR cirrus). We calibrate this effect by
proposing functional forms for the coefficients in front of the IR term which
depend on band ratios sensitive to the sSFR. These calibrations prove reliable
as a function of physical scale. In particular, they agree within 10% with the
attenuation corrections computed from the Balmer decrement on 100 pc scales.
Despite small quantitative differences, our calibrations are also applicable to
integrated galaxy scales probed by the MaNGA survey, albeit with a larger
scatter (up to 0.22 dex). Observations with JWST open up the possibility to
calibrate these relations in nearby galaxies with cloud-scale ($\sim$100 pc)
resolution mid-IR imaging.
