If it’s a zombie apocalypse, then I would assume the grid is down. Critics would say that the battery and the durability is a problem, but I don’t think it’s a problem in this time and age. So is the phone really the best source of knowledge during a zombie apocalypse?
A smartphone can potenttialy be quite useful even without cellural/internet connection. Entire wikipedia can be downloaded for offline use and many maps too. I’m sure there are plenty of survival apps that work offline too that contain a ton of useful info about edible plants, shelters, traps, plant medicine, knots etc.
None of this is useful though if you haven’t downloaded it before the apocalypse and have some kind of way of charging your device.