In this talk, we consider a wide class of functionals with the property of changing their growth and ellipticity properties according to the modulating coefficients in the framework of Musielak-Orlicz spaces. In particular, we provide an optimal condition on the modulating coefficient to establish the H"{o}lder regularity and Harnack inequality for quasi-minimizers of the generalized double phase functional with (G,H)-growth for two Young functions G and H.