Login Script

I have a login script. However i need some help with if there is a user logged in the script will redirect him rather than have them login again.

<?
session_start();
header("Cache-control: private");
include 'includes/db_connect.php';

if (!$_SESSION['username'] || !$_SESSION['email']){
$username = $_POST['username'];
$password = $_POST['password'];
$username = strip_tags($username);
$password = strip_tags($password);
$ip = $REMOTE_ADDR;
$domain = $_SERVER['REMOTE_ADDR'];

$username=strtolower($username);

if((!$username) || (!$password)){
echo "<br>You have not submitted one or more fields <br/>";
}else{

///check INFO
$sql = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password' AND activated='1' LIMIT 1");
$login_check = mysql_num_rows($sql);


///other
if ($login_check == '1'){
ini_set(session.cookie_lifetime, "3600");
session_register('username');
$_SESSION['username'] = $username;
session_register('email_address');
$_SESSION['email_address'] = $email_address;
$timestamp = time();
$timeout = $timestamp-$timeoutseconds;
$cool = gmdate('Y-m-d h:i:s');
mysql_query("UPDATE users SET lastlogin='$cool' WHERE username='$username'");
mysql_query("UPDATE users SET online='$timestamp' WHERE username='$username'");

mysql_query("UPDATE users SET ip='$domain' WHERE username='$username'");

echo "<meta http-equiv='Refresh' content=0;url=logged.php>
";

} else {
echo "wrong username/password combination or account not yet activated
";
include 'login.php';
}}}
?>

any help would be appreciated.

create a verify_auth() function returning Boolean. basicaly to check whether a) the session is created and b) whether the session details are valid. then you can bypass the login.


if( ! verify_auth() ){
//...do login script
}