You can't get an "equation" for a spline. Well, you can, IF you understand splines in some depth, but it is a bit of a piecewise mess no matter what you would do. So it is actually many equations, all in little pieces. There is no simple thing you can easily write down.
However, you can use tools to evaluate the spline at any point. That part is easy. (fnval or ppval) So why do you think you need an equation?
