popup and redirct

Hi
I have the followoing code in after record added event which works ok...

header("Location: por_details_list.php?a=return");

exit();

I need a popup message so I changed the code to get a popup and this works ok ...

echo "<script>alert('I\'m a popup window')</script>";

However when I try and combine the code I get an error message.

echo "<script>alert('I\'m a popup window')</script>";

header("Location: por_details_list.php?a=return");

exit();

Can anyone help with the code that displys a popup first and once the user click OK they are redirected.
Many thanks