Preperiodic points for rational functions defined over a global field in terms of good reduction

Let $\phi$ be an endomorphism of the projective line defined over a global field K. 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. The result is completely new in the function field case and it is an improvement of the number field case.