How to Bind Variable using Query String

CREATE OR REPLACE FUNCTION XX_GET_VALUE_F (p_person_id      NUMBER,

                                           p_column_name    VARCHAR2)

   RETURN VARCHAR2

IS

   l_column_value   VARCHAR2 (2000) := NULL;

   l_query_string   VARCHAR2 (2000) := NULL;

BEGIN

   l_query_string :=

         'declare

   l_column_value varchar2(2000);

   begin

                select '

      || p_column_name

      || ' into :l_column_val from per_people_x where person_id =  '

      || p_person_id

      || ';'

      || ' end;';

   EXECUTE IMMEDIATE l_query_string USING OUT l_column_value;

   RETURN l_column_value;

EXCEPTION

   WHEN OTHERS

   THEN

      RETURN NULL;

END;

select XX_GET_VALUE_F(70811,'LAST_NAME') from dual