SELECT EXTRACT doesn't work with variables?

I try to do something like this:

CREATE FUNCTION prova() RETURNS numeric(20,0) AS
'
DECLARE
aux TIMESTAMP;
aux2 numeric(20,0);

BEGIN
aux = ''01.01.2002 00:00:00 CET'';
aux2 = SELECT EXTRACT(EPOCH FROM TIMESTAMP aux);
RETURN aux2;
END;
' LANGUAGE 'plpgsql';

I obtain this error:

NOTICE: Error occurred while executing PL/pgSQL function prova
NOTICE: line 8 at assignment
ERROR: parser: parse error at or near "SELECT"

Is this a known bug?