在任何浏览器中在网页中间显示一条消息

Question

这是我的“登录”条件和“msg”类

<?php
$username = $_POST['username'];
$password = $_POST['password'];

if($username&&$password)
{   
    $query = mysql_query("SELECT * FROM users WHERE username='$username'");
    $numrows = mysql_num_rows($query);

    if($numrows!=0)
    {
        while($row = mysql_fetch_assoc($query))
        {
                $dbusername = $row['username'];
                $dbpassword = $row['password'];
        }

        if($username==$dbusername && md5($password)==$dbpassword)
        {
            $_SESSION['username']=$username;
            echo "<table class='msg'>";
            echo"<tr>";
            echo"<td>";
                die ("You are loged in!
                    <a href='admin_panel.php'>click here</a> to enter the <b>Admin Panel<b>.");
            echo "</td>";
            echo "</tr>";
            echo "</table>";
        }
        else
        {
            $wrongpass = "Incorrect password!";
        }
    }
    else
    {
        $exist = "That user doesn't exist!";
    }
}
?>

   .msg
   {
   margin:0 auto;
   margin-top:200px;
   font:16px;
   }

实际上我想在填充200px的网页中间显示die()警报。使用margin:0 auto属性我无法做到。有没有其他方法可以拥有它？那我该怎么办呢。