[PHP] how can I add a delay after a line of code?

I'm on a script that loads a site. I'm using header function, and I want another site to load after the one at the top. But it loads immediately after the first site, but I want users to read what's on the first site before the redirect. I want to add a delay like 7secs to allow them read before the second line loads.

<?php header('Location: http://www.site1.com/');
/*i want to add delay here*/
header('Location: http://www.site2.com/'


Thanks in advance