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