[Vince]

Does it need to be completely random, though? Because you could just use the player's unique SQL id in the number. Like the first two digits are random(100) and the last four digits are the player's unique id, padded with zeros if it's less than 1000 (%04d). This method may make it slightly obvious what's going one because the first players will have xx0001, xx0002, xx0003, etc. But it should appear more random once registered player count reaches above 100.