WordPress: Adding first/ last class to nav menu items

<?php
//Adding first and last class to nav menu items

function nav_menu_first_last( $items ) {
    $position = strrpos($items, 'class="menu-item', -1);
    $items=substr_replace($items, 'last ', $position+7, 0);
    $position = strpos($items, 'class="menu-item');
    $items=substr_replace($items, 'first ', $position+7, 0);
    return $items;
}
add_filter( 'wp_nav_menu_items', 'nav_menu_first_last' );


?>