How to SECURELY save variables?

Is it a good idea to save the game’s coins (gained through in-app purchases)and scores using playerPrefs? OR to make such variables static?

Are these methods secure to implement for server side and local scoring? and hence are they safe from any hacking cases?

EDIT

A more clearer question: How can I keep/save variables without having the using tinkering with the values OR what is the best technique for such security

You could keep track of how the coins are earned somehow. Or you make a algorithm for encrypt/decrypt the amount of coins and store this in the prefs, maybe even as strings or some other type. So in worst case for the hacker he would decrease the coins.