  We use an unprecedented sample of about 23,000 HII regions detected at an
average physical resolution of 67pc in the PHANGS-MUSE sample to study the
extragalactic HII region Ha luminosity function (LF). Our observations probe
the star-forming disk of 19 nearby spiral galaxies with low inclination and
located close to the star formation main sequence at z=0. The mean LF slope
$\alpha$ in our sample is =1.73 with a $\sigma$ of 0.15. We find that $\alpha$
decreases with the galaxy's star formation rate surface density and argue that
this is driven by an enhanced clustering of young stars at high gas surface
densities. Looking at the HII regions within single galaxies we find that no
significant variations occur between the LF of the inner and outer part of the
star-forming disk, whereas the LF in the spiral arm areas is shallower than in
the inter-arm areas for six out of the 13 galaxies with clearly visible spiral
arms. We attribute these variations to the spiral arms increasing the molecular
clouds arm--inter-arm mass contrast and find suggestive evidence that they are
more evident for galaxies with stronger spiral arms. Furthermore, we find
systematic variations in $\alpha$ between samples of HII regions with high and
low ionization parameter q and argue that they are driven by the aging of HII
regions.
