PHP's SPL has two build-in Iterators that deal with multiple iterators at once: AppendIterator and MultipleIterator. In this posting I'll cover both a bit.

He gives some sample code for each - showing how to use the AppendIterator to combine multiple iterators into one set and using the MultipleIterator to attach (not append/merge into one set) multiple iterator objects to a single, iteratable object. He als mentions a "lost" iterator, the DualIterator that never made it out of a SVN repository and into the main codebase.

The SPL AppendIterator provides a method of iterating over multiple arrays at the same time. This can be very useful when aggregating data from different sources, such as XML or RSS feeds or even multiple database sources. The flexibility and efficiency of the whole SPL suite of tools is a the disposal of the AppendIterator making manipulating data in multiple objects fast and simple with a minimum of code.

His example shows what an AppendIterator object looks like, how to use it to add new values to an array and how to filter values out of the append process with a FilterIterator object.