After numerically solving an ODE, I want to save the solutions as a .dat file. I got this from the internet net for PDES and tried by adapt it to odes using dsolve. But I get this: Error, `sol` does not evaluate to a module

PDE := diff(u(x, t), t, t)-(diff(u(x, t), x, x))+.2(diff(u(x, t), t))-1/2*(u(x, t)-u(x, t)^3) = 0;

IBC := {u(-15, t) = -1, u(15, t) = 1, u(x, 0) = tanh(x), (D[2](u))(x, 0) = -1/cosh(x)^2}; sol := pdsolve(PDE, IBC, 'numeric', u(x, t...