When the cokernel of a sheaf morphism is a sheaf?

Question

Let $\mathcal{F},\mathcal{G}$ are sheaves of abelian groups on a topological space $X$ and $\phi:\mathcal{F}\mapsto\mathcal{G}$ be a sheaf homomorphism. Then $coker\phi$ turns out to be a pre-sheaf. Does it become a sheaf under some additional natural conditions imposed on the morphism or on the sheaves?(e.g. flasque sheaves, injective morphism etc.)