Pass array literal to PostgreSQL function

I have a Postgres function which contains a select statement. I need to add a condition using a passed in variable containing an array of string values.

CREATE OR REPLACE FUNCTION get_questions(vcode text)
  RETURN return_value as $f$
DECLARE vresult return_value;

BEGIN
--snip--

SELECT id, title, code
FROM questions WHERE code NOT IN (vcode);

--snip--

questions table:

id ,title, code
1, "title1", "qcode1"
2, "title2", "qcode2"
3, "title3", "qcode3"
4, "title4", "qcode4"

How should the vcode literal be formatted in PHP and what should be the syntax of the condition?

Using PostgreSQL 9.1.1, PHP 5.3.6, pg_query_params.