I'm having a problem on how do I put primary keys whenever a user register another person. My pattern is this (batch number) + (incrementing value). For example a person named Cary register to my website and he inputted his batch number as batch 53, so his primary will become 53001. Then if another person named Josh with batch number 53 again register to my website his primary should be 53002. Then another person name Brock register but his batch is 36 so his primary should be 36001. So my solution is to get the batch number that the user inputted while they are registering then combine it with the incrementing value. My problem is how do I get the incrementing value and let the php know the next incrementing value? How do php know that the next number will be 003 or 004 or 005.