SELECT?

Hello, I have one MySQL table with about 500 rows. I need to read the table one row at a time, make some changes to data in one field and then store the changed data to another table. I'm using PHP to change the data but I have no idea how to select one row at a time from the DB table. There's one auto-increment id field in the table but the id's doesn't start from 1 and are not sequential. Any ideas or help is much appreciated! Thanks -Will