You have several misunderstandings here. First:
So what happened when you wrote those lines? First, you created DOUBLE variables, named phi_x, phi_y, phi_z. You gave them numerical values.
But then what does the syms line do? It ovewrites the existing variables, replacing them with new ones that no longer have the values you assigned. So at this point, phi_x is no longer pi/2. It is just a symbolic variable with an unknown value. It might be a complex number. It might be anything.
Next, just because you have a symbolic expression like R_x, does not mean you can then perform a direct substitution using R_x(phi_x). R_x is a symbolic expression, not a function that can be used for evaluation.
You can use tools like subs, to substitute a value for a symbolic variable. So this would work:
Or you could use a tool like matlabFunction to convert them into functions.