We study stochastic resonance in periodically driven stochastic bistable systems in terms of phase synchronization. By introduction of an instantaneous phase for the output we show explicitly the effect of phase locking between the input and output. The stochastic dynamics of the phase difference between input and output appears to be similar to that of synchronized classical self-sustained oscillators. The degree of phase coherence is estimated by employing the effective diffusion constant for the phase difference. This coherence becomes maximal for optimal noise intensities. However, phase synchronization effects can only be observed for sufficiently large magnitude of the periodic inputting signal.