posting data from sqlite to mysqldb using json - php

Question

I am trying to get JSON posted by an android application and decode it into an array to store it in my sqldb.
I have written the following php code. When I execute it, it displays [] (line 6).
<?php
include_once './db_functions.php';
//Create Object for DB_Functions class
$db = new DB_Functions();
//Get JSON posted by Android Application
$json = (isset($_POST["usersJSON"])? $_POST["usersJSON"] : '');
//Remove Slashes
if (get_magic_quotes_gpc()){
$json = stripslashes($json);
}
//Decode JSON into an Array
$data = json_decode($json);
//Util arrays to create response JSON
$a=array();
$b=array();
//Loop through an Array and insert data read from JSON into MySQL DB
for($i=0; $i<count($data) ; $i++)
{
//Store User into MySQL DB
$res = $db->storeUser($data[$i]->userId,$data[$i]->userName);
//Based on insertion, create JSON response
if($res){
$b["id"] = $data[$i]->userId;
$b["status"] = 'yes';
array_push($a,$b);
}else{
$b["id"] = $data[$i]->userId;
$b["status"] = 'no';
array_push($a,$b);
}
}
//Post JSON response back to Android Application
echo json_encode($a);
?>