如何在两个Android应用程序之间共享PHP会话？

I am using xampp to send downstream and upstream messages between two android apps. In my php server I am successfully sending data to test1.php and test2.php. But I would like to share the values in some variables in both php files. It turns out the session id is unique with each interaction, thus impossible to access the variables from the second app. test1.php looks like:

<?php 
        //session 
        session_start();

        $json = file_get_contents("php://input");
        $jsondecoded = json_decode($json, TRUE);
        $Id = $jsondecoded['ID'];  

        $_SESSION['id'] = $Id;
?>

and test2.php looks like this:

<?php 
        //session
        session_start(); 

        $myid=$_SESSION['id'];
?>

But I get id undefined error. Any help would be much appreciated.