Login Script

[Unholy Prayer]

I'm having more problems with my login script.  I'm successful logging in, but when I refresh my page or go to a different one, it says I'm logged in as someone else.  My friend just registered and when I refreshed the page it said i was logged in as his username.  This is my page header script:

<?php

$action = $_GET["action"];

$m_time = explode(" ",microtime()); 
$m_time = $m_time[0] + $m_time[1]; 
$starttime = $m_time;
session_start();
ob_start();
require_once('config.php');


include('constants.php'); 

if(!session_is_registered("username")){

      $user = "Guest";

  }

if(session_is_registered("username")){

     $user = $_SESSION['username'];

            $uidquery = mysql_query("SELECT * FROM vbb_members WHERE username='".$_SESSION['username']."'");

        $get=mysql_fetch_array($uidquery);

                 $uid=$get["uid"];


    }

 

Thanks in advance.