In the course of writing my next book, "Iterating PHP Iterators", I found something very interesting. I have a short chapter on the CachingIterator. One of the flags in the CachingIterator is FULL_CACHE. It was during my experiments with that, that I found…an anomaly.
He includes a snippet of code showing the behavior that does work, just not exactly as expected. He found that to have the values correctly cached, he had to loop through the entire iterator before trying to use it. He also answers the "just rewind() the iterator" comments with another code snippet showing it with the same behavior as before. His final example is one that does work as expected, unsetting the correct index and replacing the value as requested.
So today I learned, don't use the FULL_CACHE flag on the CachingIterator. I am not sure what the FULL_CACHE flag is supposed to do, but it doesn't seem to do anything useful at the moment. Also, it can screw things up for you.