Sorry I probably should have explained this more at the start. It's to pick topics randomly from a database but I want it to concentrate on picking the newest topics. I'm ordering the topics by latest posts and want it to pick one at random and run a script. I don't want it running the script too often on topics that are a year or two old.Maybe you should explain again what you want it to do. I thought you wanted to generate a random numbers that with each generated number have a higher probability of generating a bigger value. Since your friend suggested a matrix with a curve etc.
It would be useful for eg. a link checker but I've other ideas I want to use it on first eg. a fake account which replies to recent topics and only rarely replies and bumps an old topic.
That's why I want it to pick lower numbers more often and as the number grows the chance of it being picked decreases.
As I said your earlier example is fine for the purpose of the script and this topic is just improving the selection method and my knowledge which isn't exactly necessary but is helpful and greatly appreciated.