When you say "call" do you mean "display"? Is cars.php a page you can view with content if you browse to it?
If so I think what you are looking for is an IFRAME... the basic syntax would be:
Code:
<IFRAME SRC="http://domain.com/rotator/cars/cars.php" WIDTH="100%" HEIGHT="50"></IFRAME>
That would embed the output from the php file into your existing page.
You can search google for more options on the IFRAME tag.