Updating An Rss File Using A Php Form

Hi, I'm currently making a site for my organization but I'm stuck on this one section where I would have to make a PHP form in order for other members to post an item into my RSS file. So does anyone know a PHP script I can use in order for me and other members to input a new RSS item into my news RSS file?

one of these 3 links might be able to help you out in what your looking for

http://forums.xisto.com/no_longer_exists/
http://forums.xisto.com/no_longer_exists/
http://forums.xisto.com/no_longer_exists/

Funny it's all coming from the same site but one of them might strike a cord with what your looking for.