Algorithm ideas to determine list order/rating system?

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PHP

I want to create a dynamic list where users can add additional items to it at any point. However, I also want the items on the list to be ordered based on some sort of rating system. Any ideas on how to do this since new items can be added to the list at any time?

More info:
Users will add items to the list and users will also be able to rate the currently listed items. The list will be sorted based on the ratings the items receive.

The main problem I'm concerned with is the fact that the more recently added items would have a disadvantage in being ordered properly. For example.... Let's say an item was added a month ago and received 10 ratings within that time frame and another item that was added a day ago and received 9 ratings. If the list was ordered simply based on ratings, then item with 10 ratings would be listed before the item with 9 ratings even though it's obvious that the item with 9 ratings is more popular. Something like that.....