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 interface has no methods, its only purpose is to be the base interface for all traversable classes.