You don't need PHP to do it. This is a valid HTML tag I've used before:
<META HTTP-EQUIV="Refresh" CONTENT="10; URL=main.html">
This should be put in the <HEAD> tag of the page you want showing for 10 seconds or so. The "10" means the page will be redirected to the page in the URL field after 10 seconds ("main.html" here).
If you still want to use PHP, you can redirect the user to another site using
header("Location: somelink.php");
...but only if there are no HTML tags prior to this line in your code. To do it after a certain amount of time, it looks similar to the code inside the first tag up there:
header('Refresh: 10; URL=http://www.example.com');
Hope this helps.
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