How do I add a call to a PHP script from a template?

[ataraxia]

How can I add a call to, say, a banner rotation script to my footer template?

My friend says we need to set contents of variable, which we set in Plugin (simple one $myvar = 'MyVarContents' which is executed on some event/action through official Plugins system. And now we need to get this variable contents in to one of default templates... There are no hacks or other modifications of code.

How does one do this?

thanks!