To triponline:
Mordecai has already answered the post, which is to use the header() function. I anticipate that your first problem with the header() function will be some error messages about headers already being sent. You'll see.
When it happens, just be aware that nothing can be outputted BEFORE the header call.
For example, not acceptable:
<?php
echo"You're logged in now!";
header("Location: post_login_page.php");
?>
That echo statement outputs, so you will get the header error messages that I described. Do not put anything before the header call that will output.