I need to use a php include in my template. How?

[Emil]

I need to use a php include in my footer template to show some ad links.

But I can't just add the php include into the footer template because it doesn't work.

I need to include this:

<? include ("display.php"); ?>

Is there any way I can add a php include to my template? I heard something about using plugins, but I have no idea how to.

Please help. Thanks!