Writing complex equations in PHP is big deal so it would be easier if I could write them as "WYSIWYG" i.e. with MS Mathematics and export them instead of coding them. I checked out if I could use MATLAB but it can only export to C which would mean I would have to rewrite nearly everything...
If Runge-Kutta and FFM simulation is something to you, then you might understand the complexity...
An example of MS Mathematics syntax:
gives xi(t_(n+1))-v_xi(t_(n+1))*(DELTA_(t/2))=xi(t_n)+v_xi(t_n)*(DELTA_(t/2))
(Errors may be in it because of the inconvenient way of copying this)