SELECT * FROM register WHERE username != 'admin'
However, if I were doing it I would add an intenger column to the table called 'admin' with a default of 0 (not an admin account), then set the value to a non-zero number for each admin account. This gives you the ability to add additional admin accounts, possibly with different levels of responsibility and thus different values, while never having to worry about changing the PHP code if/when you add/change accounts in the DB. Then to get all normal users you could simply query:
SELECT * FROM register WHERE admin = 0