On DZone.com there's a recent post from Evert Pot talking about some guidelines for generating XML including a brief PHP example.

Over the last little while I've come across quite a few XML feed generators written in PHP, with varying degrees of 'correctness'. Even though generating XML should be very simple, there's still quite a bit of pitfalls I feel every PHP or (insert your language)-developer should know about.

His suggestions for better XML include:

• You are better off using an XML library (like xmlwriter)
• Understand Unicode
• CDATA is never a solution
• Be verbose
• Be careful with entities

Federico has posted a list of suggestions on things to look out for and to consider when you're refactoring your code.

Refactoring neither fixes bugs nor adds new functionality, though it might precede either activity. Rather it improves the understandability of the code and changes its internal structure and design, and removes dead code, to make it easier to comprehend, more maintainable and amenable to change. Refactoring is usually motivated by the difficulty of adding new functionality to a program or fixing a bug in it.

He's broken it out into a few topics with the suggestions underneath - things like looking at the "Big Picture", avoiding "Extreme Abstraction" and ensuring that your "Error Handling" is up to where it should be.