Adding PHP code to the header

I have a php script that I run with the rest of my layout, and I can't figure out how to get it to use script if I add php code to the header.php file in the administration section of the guestbook. Does anyone know how to allow php code to be run when you put it in those files, or do I need to do something else to get it to work?

Ok, well I added the php file that included the function its going to do, then I went into the language file and made one like this:

so that when that variable is run it should display the countdown date. It shows it, but the problem is, is that it puts it at the top, its not putting it where I want it.

Any ideas on why it wont go where I put the $Lang[DaysLeft] in the header.php file?

you place in it the index.php, addentry.php and the comment.php file. but where is the rest of the code?