侦测到12级灵能波形:用session编写用户验证页出现错误

我是这样写的登陆页的代码
---------------------------------
<?
require_once("../require/global.php");//数据库连接

session_start();
$username = addslashes(trim($_POST['username']));
$password = addslashes(trim($_POST['password']));

$sql = mysql_query("select * from ".$db_prefix."admin where user='$username' and pwd='$password'");

if($r=mysql_fetch_array($sql)){
$_SESSION[username]=$r[$user];
$_SESSION[password]=$r[$pwd];
echo "登录成功！";
}else{
session_destroy();
echo "对不起，帐号和密码不匹配！";
}
?>
------------------------------------------------

可是在运行时，我登陆如果是错误的帐户和密码，echo "对不起，帐号和密码不匹配！"; 这句错误显示是正常的。

但是在输入正确时，却会报错，以下为错误页信息
--------------------------
登录成功！ Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively. in Unknown on line 0
---------------------------------------------------

不知道为什么会出现这个错误，请高指导，谢谢。