How do I convert my "hack" to a plugin / addon???

[3CS]

I think this is a similar question so rather than start a new thread...

I have some working php code that I don't know how to incorporate into my forum. Just under my banner I want some php code to write out some javascript - one of 2 scripts will run and the php chooses one randomly :

PHP Code:

<?
$random_url = array("http://www.tickermyfeed.com/feed.php?HRMV8tbSD0", "http://www.tickermyfeed.com/feed.php?P2JveHdpZHRoP");
srand(time());
$random = (rand()%2);
print("<script language=\"javascript\" src=\"$random_url[$random]\"></script>");
?>

The real http addresses there are miles long so I've abbreviated them.
I was hoping to just add this php code in under the Admin CP, "Common Templates > header".
What exactly do I need to do to make this work?
I've just read various tutorials but can't see what I need.

Thanks in advance for any ideas :up: