XML converter needed

I need a server side script (PHP) that can spider an external xml feed and then convert that feed to a different standard then save it to a location on our server. This script may already exist, but I haven't found one yet. For example, I have an ebay feed that use ebay item names and descriptions. Using this script I create a set of conversion standards, such as convert item name DATE to DATE1. So it will find the field DATE in the ebay feed, convert that to DATE1 and save the new feed. The script will rename each item for all items in the feed and then save it. It must be able to handle LARGE xml feeds without freezing or crashing, it must be able to run via cron and it must report successful conversion and all errors via email. Should take a fast coder about 2 days of work.