Preperiodic points for rational functions defined over a rational function field of characteristic zero

Let k be an algebraic closed field of characteristic zero. Let K be the rational function field K = k(t). Let $\phi$ be a non–isotrivial rational function in K(z). We prove a bound for the cardinality of the set of K–rational preperiodic points for $\phi$ in terms of the number of places of bad reduction and the degree d of $\phi$.