header redirect

[ifyrlucky]

I want one domain address to go to the index.php and the domain alias to go to another page within the website. I am told this is done with a header redirect? I have been given this code

<?php
header("Location: [a href=\"http://www.example.com/");\" target=\"_blank\"]http://www.example.com/");[/a] /* Redirect browser */

/* Make sure that code below does not get executed when we redirect. */
exit;
?>

But I'm not sure exactly how it works or how to modify it to my needs, specifically where I need to type in the domain name alias and where I need to type in the url for the page I want it directed to.