[~, coeffs] = unmkpp(pp);
Splines are not a single equation in the sense of a single polynomial that works from beginning to end. Splines are piecewise polynomial, and if you want a single equation, it would have to be constructed as piecewise.
The breaks (places each segment is valid) are returned as the first output of unmkpp.
"Polynomial coefficients, returned as an L-by-k matrix with each row coefs(i,:) containing the local coefficients of an order k polynomial on the ith interval, [breaks(i),breaks(i+1)]."
So create a vector
phi>= breaks(1:end-1) & phi<breaks(2:end)
and put those into individual cells, and do your matrix multiplication with phi powers and slice the results by row into cells, interleave the cells, piecewise() of cell expansion to get the final expression.
(In practice you need to repair the last upper bound to <= instead of < )