Sufficient API Response for Mobile App

I’m creating an API for my mobile app with Laravel – Lumen and i have the following scenario which i don’t know what is the most sufficient way to return the response to my Android / IOS App,

lets assume that the user is trying to sign in (same end point):

Case 1 : the user provide the correct data for the sign in.

Case 2 : the user sign in with wrong password.

Case 3 : the user is not exist and have to sign up first.

My question is what should my API response look like in order i can differentiate those cases in my App logic and to move forward accordingly (I need to know which case is occurred ) ?

Answer

Your response would be below –

Use this response if request validated successfully. 
{"status":"OK", "message":"success"}

Use this response if user not exist.
{"status":"FAIL", "message":"not_register"}

Use this response for invalid credentials.
{"status":"FAIL", "message":"invalid_credentials"}