Have a read through this SitePoint article which explains how to migrate over from the old (and now deprecated) mysql_* extension over to PDO. Deleting by firstname is a very, very bad idea, say you've got 50 people in the database with the firstname John and another 50 with the firstname David. If you go and delete by firstname David and John, you'll end up deleting 100 people from the database when you probably only meant to delete two people, one called John and another called David.
By using an interger field (a length of 11 should cover any number of new members - very few websites will have in excess of 99,999,999,999 members) set as an auto increment you'll have a value that is unique. A members email address will also serve as a unique id provided that you ensure that no registrant can use an email address that already exists in either the registration or user tables. The use of email however does have a disadvantage as a person might change email address in the future.