XML with PHP/MySQL

Alright. So from what I understand, XML is a markup language that allows you define your own tags and output them how you like. How does this complement PHP? If PHP allows you to take data and output them how you like, then how does XML augment PHP? I know it's a dumb question to people who know the answer, and I'm not arguing the fact, I'd just like to know how XML fits into web programming as a whole.

Thanks!