Add custom classes to WordPress menus

Add class to menu

// Add extra classes for the first and last items in all WordPress menus
add_filter( 'wp_nav_menu_objects', function ( $items ) {
   if ( ! empty( $items ) ) {
      $items[1]->classes[] = 'menu-item-first';
      $items[ count( $items ) ]->classes[] = 'menu-item-last';
   }

   return $items;
} );

Append to menu

<?php
/**
* Add a custom link to the end of a specific menu that uses the wp_nav_menu() function
*/
add_filter('wp_nav_menu_items', 'add_admin_link', 10, 2);
function add_admin_link($items, $args){
    if( $args->theme_location == 'footer_menu' ){
        $items .= '<li><a title="Admin" href="'. esc_url( admin_url() ) .'">' . __( 'Admin' ) . '</a></li>';
    }
    return $items;
}