The Web Mozarts blog has an interesting new post today that talks about using annotations in your PHP code to define the attributes in resulting XML that could be generated dynamically from your objects.
Annotations have become a popular mechanism in PHP to add metadata to your source code in a simple fashion. Their benefits are clear: They are easy to write and simple to understand. Editors offer increasing support for auto-completing and auto-importing annotations. But there are also various counter-arguments: Annotations are written in documentation blocks, which may be removed from packaged code. Also, they are coupled to the source code. Whenever an annotation is changed, the project needs to be rebuilt. This is desirable in some, but not in other cases.
They focus in on Symfony-based applications as a good base to work from (as they've pushed to have annotations work in the code for things like routing and data type definition). He starts with an example Doctrine class - a "best buddy" for Symfony as far as annotations go - and how the annotations define the different properties. He also includes an example of the XML output of the same definition generated through an "AnnotationReader" instance. He talks about having multiple XML documents representing one object with different annotations put in each, including the XML output. The post finishes with some advantages including the ease of validation by XML-friendly tools looking to interface with the application.