php/mysql - getting ID of query

This is as much a mysql question as it is php. What is the most reliable way to retrieve an auto_increment key/id for a query you just inserted? In other words, I compile all my data, insert it into a new row, and now I want to retrieve the ID it created for it in the ID/key field. Without turning around and searching for the values I just inserted, is there a way to get the ID? The ID is the only field that is guaranteed to be unique in each data set, which makes it questionable to search for the data you just inserted, unless you searched for the *whole* data set. Thanks John