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.
注意: Internal (built-in) classes that implement this interface can be used in a foreach() construct and do not need to implement IteratorAggregate or Iterator.
注意: This is an internal engine interface which cannot be implemented in PHP scripts. Either IteratorAggregate or Iterator must be used instead.
This interface has no methods, its only purpose is to be the base interface for all traversable classes.