How to use multiple sessions for four different users of a web application in PHP?

Question

<?php
    $session_uid=$_SESSION['uid'];
    // Session Private
     if(!empty($session_uid))
    {
       $uid=$session_uid;
      $login='1';
    }
    else if($_GET['username'] || $_GET['msgID'])
     {
    $uid=$obj->User_ID($username);
    $login='0';
    }
    else
    {
     $url=$base_url.'log.php';
 
     //$url=$base_url.'login.php';
     header("location:$url");
     }

       ?>