select reverse('ganesh') from dual

SQL> CREATE OR REPLACE FUNCTION fun_reverse (par_str IN CHAR)
  2  RETURN CHAR
  3  IS
  4    retval VARCHAR2(100);
  5  BEGIN
  6    FOR i IN REVERSE 1 .. LENGTH(par_str) LOOP
  7      retval := retval || SUBSTR(par_str, i, 1);
  8    END LOOP;
  9
 10    RETURN (retval);
 11  END;
 12  /

Function created.

SQL>
SQL> SELECT fun_reverse ('A quick brown fox jumps over a lazy dog.') result
  2    FROM dual
  3  UNION ALL
  4  SELECT REVERSE('A quick brown fox jumps over a lazy dog.')
  5    FROM dual;

RESULT
---------------------------------------------------------------
.god yzal a revo spmuj xof nworb kciuq A
.god yzal a revo spmuj xof nworb kciuq A

SQL>