It is possible to wait up to 15 minutes for the inter-terminal shuttle bus (I'm not sure what the official schedule is, but I've waited that long). In addition, PHL is a very busy airport that has frequent delays for all sorts of reasons, including delays between landing and parking at the gate. So I would say 45 minutes is cutting it rather fine. You might make it, but there's an equal-to-greater chance that you won't. Take an earlier inbound flight and rest easy.