474151 PHP XML Parser Script

I need a simple PHP XML parser script that can handle parsing large files (over 20MB) without PHP timeouts and memory issues. I have looked at XMLReader and other PHP parsers available. I need a script that will parse an XML file and then save the results to a database. Functions: - Check to see if post already exists (check post ID number in feed) and if it exists check to see if it is updated (compare the update date field in the feed), if update date is newer than update database otherwise skip. - Check to see if post category exists, if not create category and then add post to that category. If this can not be run from the front end it can also be set to be run from a cron job. This is not to complicated and shouldn't take to long. Please PM questions. Needs to be done today.