9i> declare
  2     v long;
  3  begin
  4     select reverse('hello') into v from dual;
  5     for i in reverse 1 .. 3 loop
  6        dbms_output.put_line( i || ' ' || v);
  7     end loop;
  8  end;
  9  /
3 olleh
2 olleh
1 olleh

PL/SQL procedure successfully completed.

SQL> DECLARE
  2     v_reverse VARCHAR2(4000);
  3  BEGIN
  4     SELECT REVERSE('QWERTY') INTO v_reverse FROM dual;
  5     DBMS_OUTPUT.PUT_LINE(v_reverse);
  6  END;
  7  /
YTREWQ

PL/SQL procedure successfully completed.

SQL> CREATE OR REPLACE FUNCTION my_reverse (p_str VARCHAR2) RETURN VARCHAR2 AS
  2    v_str VARCHAR2(4000);
  3  BEGIN
  4    FOR i IN <B>REVERSE</b> 1..LENGTH(p_str) LOOP
  5        v_str := v_str || SUBSTR(p_str, i, 1);
  6    END LOOP;
  7    return v_str;
  8  END;
  9  /

Function created.

SQL>
SQL> SELECT my_reverse('QWERTY') FROM dual;

MY_REVERSE('QWERTY')
--------------------------------------------------------------------------------
YTREWQ

SQL> EXEC DBMS_OUTPUT.PUT_LINE( my_reverse('QWERTY') );
YTREWQ

PL/SQL procedure successfully completed.