So my website has an RSS feed, however it is built with .php rather than somethign like .xml like it should be. I am trying to make a valid .xml RSS feed so that I can use the RSS feed on twitter, facebook, and it would just be easier to be seen on search engines and the like adhering to the standards.

I am not very good at the whole HTML or coding thing, however, I have read about how RSS feeds work and pretty much every single tutorial is the same thing--it shows you how to make an RSS feed that you manually update.

What I am trying to accomplish is an RSS feed that instantly updates as I post content. I am not quite sure, but I believe that would mean I would edit something in between the <item></item> tags. Does anyone have any insight on how to do this?

use ajax scripting with intervall time

It doesn't matter .php or .xml.
You can use php to build dynamic RSS, but at first send proper header information:

header('Content-type: application/rss+xml');

Then write your rss/xml information from DB.