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.

<?php
    if( !is_array( $items ) && !$items instanceof Traversable )
        //Throw exception here
?>

NOTE:  While objects and arrays can be traversed by foreach, they do NOT implement "Traversable", so you CANNOT check for foreach compatibility using an instanceof check.

Example:

$myarray = array('one', 'two', 'three');
$myobj = (object)$myarray;

if ( !($myarray instanceof \Traversable) ) {
    print "myarray is NOT Traversable";
}
if ( !($myobj instanceof \Traversable) ) {
    print "myobj is NOT Traversable";
}

foreach ($myarray as $value) {
    print $value;
}
foreach ($myobj as $value) {
    print $value;
}

Output:
myarray is NOT Traversable
myobj is NOT Traversable
one
two
three
one
two
three

The PHP7 iterable pseudo type will match both Traversable and array. Great for return type-hinting so that you do not have to expose your Domain to Infrastructure code, e.g. instead of a Repository returning a Cursor, it can return hint 'iterable':
<?php
UserRepository::findUsers(): iterable
?>

Link: http://php.net/manual/en/migration71.new-features.php#migration71.new-features.iterable-pseudo-type

Also, instead of:
<?php
    if( !is_array( $items ) && !$items instanceof Traversable )
        //Throw exception here
?>

You can now do with the is_iterable() method:
<?php
    if ( !is_iterable( $items ))
        //Throw exception here
?>

Link:  http://php.net/manual/en/function.is-iterable.php

Note that all objects can be iterated over with foreach anyway and it'll go over each property. This just describes whether or not the class implements an iterator, i.e. has custom behaviour.