Base point (0,0,6) after rotation will lie in XZ plane with coordinates
(x0, y0, z0) = (-6*sin(Fi), 0, 6*cos(Fi))
normal vector
n = (A,B,C) = (-sin(Fi), 0, cos(Fi))
so new plane equation is (for explanation see beginning of the article)
A*(x-x0)+B*(y-y0)+C*(z-z0)=0
or
-sin(Fi)*x + cos(Fi)*z - 6 = 0