While you cannot implement this interface, you can use it in your checks to determine if something is usable in for each. Here is what I use if I'm expecting something that must be iterable via foreach.
if( !is_array( $items ) && !$items instanceof Traversable )
//Throw exception here
The Traversable interface
(No version information available, might only be in SVN)
Interface to detect if a class is traversable using foreach.
This is an internal engine interface which cannot be implemented in PHP scripts. Either IteratorAggregate or Iterator must be used instead. When implementing an interface which extends Traversable, make sure to list IteratorAggregate or Iterator before its name in the implements clause.
This interface has no methods, its only purpose is to be the base interface for all traversable classes.