Handling different php responses after a javascript http post request

[MarkV]

I am developing an android application that uses the PHP/MySQL to send data from app to server in order to register/login users. I already wrote the Javascript and PHP files to send and receive JSON data an insert it into MySQL database. The problem I have is how to handle different responses from PHP. Exemple:\[code\]<?php //decode json object $json = file_get_contents('php://input'); $obj = json_decode($json); //variables $user_firstname = $obj->{'user_firstname'}; $user_lastname = $obj->{'user_lastname'}; $user_email = $obj->{'user_email'}; $user_password = $obj->{'user_password'}; if([variables] != null){ //check for an existing email in db mysql_query("Check if email exists"); if(email exist){ //pass a response to java file return user_exists; die(); }else{ //success return success; } }?>\[/code\]All I want is to handle those different return values in order to interact with the user inside the android app.