ref id in mysql/php

Hi I have a table with id auto increment , I have another field is ref_id, i want to add same numeric data in this field for every 5 row & after every 5 row it will auto crease the no#.
Is this possible, if yes how can I do this?

Here below , what I want to do?

Mysql-table

id---ref_id
1----55
2----55
3----55
4----55
5----55
6----56
7----56
8----56
9----56
10----56