[SQL] One Parameter And Few Values.

Hello!
I have tables "forum_topics" and "forum_categories":
forum_topics:
-topic_id
-topic_title
-topic_cat_id
-topic_user_id
-topic_date
forum_categories:
-cat_id
-cat_name
 
For example: I have 5 categories in table forum_categories and by this query I can display topics from 2nd and 3rd category only. So I'm using this query:
SELECT topic_title FROM forum_topics WHERE topic_cat_id IN (2;3)
Now I want to parametrized this query by one parameter, so:

SELECT topic_title FROM forum_topics WHERE topic_cat_id IN (@param)
And I want to input this parameter lik in the query first - so @param = "2;3"
But I have a problem to make this works, can someone explain me, how to solve this problem ? Thanks.