  This paper concerns Hopf's boundary point lemma, in certain $C^{1,Dini}$-type
domains, for a class of singular/degenerate PDE-s, including $p$-Laplacian.
Using geometric properties of levels sets for harmonic functions in convex
rings, we construct sub-solutions to our equations that play the role of a
barrier from below. By comparison principle we then conclude Hopf's lemma.
