Interface to detect if a class is traversable using foreach.
Abstract base interface that cannot be implemented alone. Instead it must be implemented by either IteratorAggregate or Iterator.
Internal (built-in) classes that implement this interface can be used in a foreach construct and do not need to implement IteratorAggregate or Iterator.
This is an internal engine interface which cannot be implemented in PHP scripts. Either IteratorAggregate or Iterator must be used instead.