For example if I have z = f(x,y), then dz = df/dx * dx + df/dy * dy. Then it is clear that dz is a linear approximation of change of z. (I don't know what is a differential analytically) But what is the differential of a function in integral $$y(s) = \int_0^s f(x) \,dx$$,
For example, under the follow equations, what is d$\phi$?
It may be a dumb questions, but I can't see the differential and don't actually know what it is.