A slight correction to brcontainer's polyfill, which prevents errors on a non-object in a non-blocking way, and also corrects the issue of  the conditional checking "file_exists" instead of the correct "function_exists":

if ( !function_exists(  'is_iterable' ) )
{

    function is_iterable( $obj )
    {
        return is_array( $obj ) || ( is_object( $obj ) && ( $obj instanceof \Traversable ) );
    }

}

The original answer would not have resolved correctly, because it was looking for a file instead of a function, and the provided method would error if given a non-iterable non-object value such as false.