SQL> select reverse(78987345) from dual;
                                 REVERSE(78987345)
--------------------------------------------------
               -0000000000000000000000000000000000

1 row selected.

SQL> ed
Wrote file afiedt.buf

  1  declare
  2   inputstring varchar2(100) :='78987345';
  3   outputstring varchar2(1);
  4   n number;
  5  begin
  6  for n in 1..length(inputstring)
  7   loop
  8    select substr(inputstring,length(inputstring)-n+1,1) into outputstring from dual;
  9    dbms_output.put(outputstring);
 10   end loop;
 11    dbms_output.put_line(' ');
 12* end;

SQL> /

54378987

PL/SQL procedure successfully completed.

SQL> ed
Wrote file afiedt.buf

  1  declare
  2     input number :=78987345;
  3     output number(1);
  4    n number;
  5    begin
  6    for n in 1..length(input)
  7     loop
  8      select substr(input,length(input)-n+1,1) into output from dual;
  9      dbms_output.put(output);
 10     end loop;
 11      dbms_output.put_line(' ');
 12*  end;
SQL> /
54378987

PL/SQL procedure successfully completed.