Anthony Ferrara has a new post to his blog sharing some inconsistencies with iterators that he discovered as discussed with a fellow developer - why some iterators only accept Iterator arguments and others don't.

We were talking about why some of the SPL Iterators accept only an Iterator as the constructor argument (Such as LimitIterator), and others accept either an Iterator or an IteratorAggregate as the argument (Such as IteratorIterator). Feeling that this would be a useful feature to add (having all of them accept an IteratorAggregate), I opened up the PHP source and started looking at how hard of a change this would be. What I found was... Interesting...

He shares some of the C code he came across in his investigation including a "WTF" moment when he found a case statement for DIT_IteratorIterator in a constructor. Because of some of the logic in this constructor, the inputted iterator is "cast down" to a class. This is shown in a few code examples comparing simple iteration objects and arrays and how it seems to be able to bypass class inheritance to use methods from other classes.