The code you posted will take the last values of ‘phi_sync’ and ‘phi_acc’ to calculate ‘phi’. (Also, MATLAB is case-sensitive, so ‘Phi_acc’ is a different variable from ‘phi_acc’, and will throw an error if you haven’t defined it.)
If you want to add the two vectors of the same size, just use:
phi = phi_sync + phi_acc;
