If you set your DB table's primary key to auto increment, you aren't going to be able to set a fixed length of numbers and pad them to the left with zeros. If you want to display it in that format (00000001 vs 1) then you'll have to handle that in your application.
Second that. I'd use normal autoincrement to store integers and provide LPAD in your SELECT queries OR use php's str_pad when you retrieve the data. Don't make the DB work too hard ;)