function doLogin($username,$password) {
    if($_SERVER['SERVER_NAME'] == URL) {
        $find_user = mysql_query("SELECT * FROM ".USERS_TABLE." WHERE username = '$username' AND password = '$password' LIMIT 1");
        if(mysql_num_rows($find_user) == 1) {
            $user = mysql_fetch_array($find_user);
            if($user['active'] == 1) {
                $update_login = mysql_query("UPDATE ".USERS_TABLE." SET last_login = '".time()."',login_ip = '".$_SERVER['REMOTE_ADDR']."', WHERE id = '".$user['id']."'");
                $_SESSION['id'] = $user['id'];
                mysql_free_result($find_user);
            } else {
                $login_error = "Your account has not been activated yet.";
            }
        } else {
            $login_error = "Wrong username/password.";
        }
    } else {
        die("You do not have permission to login to this site.");
    }
}

function checkLogin() {
    if($_SESSION['id'] != '') {
        $user = mysql_fetch_array(mysql_query("SELECT * FROM ".USERS_TABLE." WHERE id = '".$_SESSION['id']."' LIMIT 1"));
        if($user['login_ip'] == $_SERVER['REMOTE_ADDR']) {
            $expired = $user['last_login'] + 600;
            if(time() >= $expired_time) {
                session_destroy();
                header('Location: index.php');
            } else {
                $update_login = mysql_query("UPDATE ".USERS_TABLE." SET last_login = '".time()."' WHERE id = '".$user['id']."'");
            }
        } else {
            session_destroy();
            header('Location: index.php');
        }
    }
}