Eric Lamb came up against an interesting situation recently and found that PHP's Reflection API was the best thing to come to his rescue.
The perfect problem where the Reflection API appears to be the perfect solution. For me this came up a couple months ago while I was working on a European zip code radius project that had to be built using one of those obfuscated and ill documented 3rd party commercial programs (nothing worse than when the platform is forced on you). So, I had to use this program that was intentionally encoded to prevent me from doing what I wanted to do. I couldn't even look at the code; it was completely obfuscated.
To illustrate his point, he gives examples of two built-in classes that PHP has and the result of their reflected output - SimpleXMLElement and DateTime. He shows how to get the structure of the class in general and how to use some of the more specific functions to get things like properties, methods and constants for the class. This is perfect for those undocumented features and isn't too far of a jump away from building out your own documentation.