Include files based on category ID - will this work?

[90210]

What I'm trying to do is to include file for each category so it would look like this:
for
cat_id = 2
{include file="add/{$category.id}.php"} (thus include add/2.php)
cat_id = 3
{include file="add/{$category.id}.php"} (thus include add/3.php)

and so on...
right now I get : Warning: Smarty error: unable to read resource: "{Array.id" in /home/sites/domain.com/smarty/Smarty.class.php on line 1095
.php"}

what am I doing wrong?