Either redirect the user using the header function and allow them to continue back or use javascript to pop open a window that the user can close.
haven't tested it, but try placing this sample javascript on the first page the user sees.
<HEAD>
</HEAD>
<SCRIPT LANGUAGE="JavaScript1.1">
function popup(url, name, height, width)
{
var popwin;
var opts = "toolbar=no,status=no,location=no,menubar=no,resizable=no";
opts += ",height=" + height + ",width=" + width + ",scrollbars=yes";
popwin = window.open("", name, opts);
popwin.focus();
popwin.location = url;
}
</SCRIPT>
<BODY>
html & content goes here
place this at the top or at the bottom of the page:
<SCRIPT FOR=window EVENT=onload LANGUAGE="JScript">
popup("message1.php","",500,400);
</SCRIPT>
</BODY>