@vv I am sorry. Here is the whole problem

pde := diff(theta(z, p), z, z)+lambda(p)^2*theta(z, p)+p*lambda(p)^2*(sin(theta(z, p))-theta(z, p))

subs(p=0,pde)

pde := diff(theta(z, 0), z, z)+theta(z, 0)   : PDE converted to ODE because of substitution

ibc:={theta(0,0)=beta,D[1](theta)(0,0)=0}; : Boundary Conditions for z=0

pds := pdsolve(pde,[ibc],numeric,time=z,range=0..beta);  : Cannot solve this by dsolve since it is identified as pde

The error: 

Error, (in pdsolve/numeric/process_PDEs) PDEs can only contain dependent variables with direct dependence on the independent variables of the problem, got {theta(z, 0)}

I want to get a solution to the linear equation on substituting p=0, and then taylor expand the original pde about p=0 to get the solution for p=1. So I cannot substitute theta[0] right in the beginning. Is there a way to manage these pdes which become odes?

Thank you. 

@vv I am trying to substitute theta(z,0) as theta[0](z) further down in the maple sheet but the terms within the differential do not get substituted for theta[0](z). Just the independent theta term gets substituted. I cannot do that from the beginnning because I need to taylor expand about the 0