Angela,
While I'm certainly no XML expert, I think you may have a slight misconception about what XML is. Though there are dozens of uses for XML, it's used most often to store data. It's not a programming language (or interpreted language like PHP) -- it's a markup language, hence the ML bit. XML most often is used as a markup language to categorize and describe data. For example, I could have an XML file listing a dozen users, their e-mail addresses, their age, their height, and their eye color. That sort of thing. As such, you don't "run" XML files. They're usually read, in the same sense that Word files or GIF files are read, by a program (and as such there really aren't any "requirements" for using XML). This is where XML and PHP intersect -- PHP has a number of functions for reading and parsing XML files. You can read about PHP's XML support here:
http://www.php.net/manual/en/ref.xml.php
As far as editing XML files, you can do so in an ordinary text editor -- I'd recommend TextPad (www.textpad.com) -- or you can use PHP's various XML functions to do so. I'd also recommend reading more about XML -- there's lots of information to be found on the W3C's XML page: http://www.w3.org/XML/
