Function to compare an array with another array

Shu! I need some help here. I have a script where I want to compare two arrays with eachother, and return one value from that does not exists in both arrays. I have an array of pictures id number created from a database query. I use sessions to save id numbers of pictures into an array. I want the first array to be compared to the second and return the first key/value-pair that are not found in the session array... Is there a way I can do this? Thankful for any help! /Broder B