declare variable in udf

Hello,

How do I declare a variable after BEGIN?
I want to declare it in the if statement.

DROP FUNCTION IF EXISTS tmp_test(integer);
CREATE FUNCTION tmp_test(
p_id integer
)
RETURNS text
AS $$
DECLARE the_return_value text;
BEGIN
DROP TABLE IF EXISTS temp_test_table;
CREATE TEMP TABLE temp_test_table(
some_value text
);
INSERT INTO temp_test_table
SELECT data FROM table WHERE id=p_id;
SELECT INTO the_return_value some_value FROM temp_test_table;

IF 1=1 THEN
--how do I declare a variable here? it this possible?
RAISE NOTICE 'this is a notice';
END IF;

RETURN the_return_value;

END;
$$ language plpgsql

Best,
Peter