how do I sort a multiple assoc array?

I have the following array structure: $stats[$i]["some_string"] $i is around is read from a file and numbers at about 150 right right now. "Some string" includes user_id, ip_addr, date and a few others, all containing the pertinent data. I want to sort this array according to the user_id. I've seen examples using the various sort() functions using associative arrays, but none with multiple assoc arrays. Eeeek. I don't know what to do. Any input? - Craig Simons