gistfile1.php

<?php

// Return the page id by slug
// -------------------------------------------------------------
function get_ID_by_slug( $page_slug ) {
  $page = get_page_by_path( $page_slug );
  if ( $page ) {
    return $page->ID;
  } else {
    return null;
  }
}

// Return if we're in the Projects section
// -------------------------------------------------------------
function is_projects() {
  return ( is_singular( 'project' ) || is_post_type_archive( 'project' ) );
}

// Return the root parent ID of a given post
// -------------------------------------------------------------
function get_root_parent_id( $post_id = null ) {
  if ( ! $post_id ) {
    global $post;
    $post_id = $post->ID;
  }
  global $wpdb;
  $parent = $wpdb->get_var( "SELECT post_parent FROM $wpdb->posts WHERE post_type='page' AND post_status='publish' AND ID = '$post_id'" );
  if ( $parent == 0 ) return $post_id;
  else return get_root_parent_id( $parent );
}

// Remove wp_nav_menu() containers
// -------------------------------------------------------------
function my_wp_nav_menu_args( $args = '' ) {
  $args['container'] = false;
	return $args;
}

add_filter( 'wp_nav_menu_args', 'my_wp_nav_menu_args' );

// Remove all wp_nav_menu() classes (and add .is-current)
// -------------------------------------------------------------
function my_wp_nav_strip_classes( $a ){
  return ( in_array( 'current_page_item', $a ) ) ? array( 'is-current' ) : array();
}

add_filter( 'nav_menu_css_class', 'my_wp_nav_strip_classes', 10, 2 );

// Add special classes to wp_nav_menu
// -------------------------------------------------------------
function my_wp_nav_special_classes( $classes, $item ){
  if( $item->object_id == get_root_parent_id() || $item->object_id == get_ID_by_slug( 'project' ) && is_projects() ) {
    $classes[] = 'is-root-parent';
  }
  return $classes;
}

add_filter( 'nav_menu_css_class' , 'my_wp_nav_special_classes', 10, 2 );

// Remove wp_nav_menu() IDs
// -------------------------------------------------------------
function my_wp_nav_strip_id() {
  return '';
}

add_filter( 'nav_menu_item_id', 'my_wp_nav_strip_id' );