Smarty is a real pain in the ass when it comes to includes and PHP.
The only way I found to do it was like this:

{include_php file="/home/http/yoursite.com/yourfile.php"}
So, you use the path to the file, and you make whatever you want to include into its own separate file.
We found that we had to do this because we were just trying to get ordinary php stuff to work, and it wouldn't.