Takes custom classes out of the list item and adds them to the anchor element in WordPress menus. Perfect for font icons

functions.php

<?php

add_filter( 'nav_menu_link_attributes','cameronjonesweb_move_custom_menu_item_class_to_anchor_element', 10, 4 );
add_filter( 'nav_menu_css_class', 'cameronjonesweb_remove_custom_menu_item_class_from_li_element', 10, 4 );

/**
 * Get the custom item menu classes and add them to the anchor element
 *
 * @link https://cameronjonesweb.com.au/blog/how-to-move-the-custom-menu-item-classes-to-the-anchor-element/
 * @param array    $atts The HTML attributes applied to the menu item's `<a>` element, empty strings are ignored.
 * @param WP_Post  $item  The current menu item.
 * @param stdClass $args  An object of wp_nav_menu() arguments.
 * @param int      $depth Depth of menu item. Used for padding.
 * @return array
 */
function cameronjonesweb_move_custom_menu_item_class_to_anchor_element( $atts, $item, $args, $depth ) {
	$custom_classes = get_post_meta( $item->ID, '_menu_item_classes', true );
	if ( ! empty( $custom_classes ) ) {
		$atts['class']  = empty( $atts['class'] ) ? '' : $atts['class'] . ' ';
		$atts['class'] .= implode( ' ', $custom_classes );
	}
	return $atts;
}

/**
 * Remove the custom classes from the list element
 *
 * @link https://cameronjonesweb.com.au/blog/how-to-move-the-custom-menu-item-classes-to-the-anchor-element/
 * @param array    $classes The CSS classes that are applied to the menu item's `<li>` element.
 * @param WP_Post  $item    The current menu item.
 * @param stdClass $args    An object of wp_nav_menu() arguments.
 * @param int      $depth   Depth of menu item. Used for padding.
 * @return array
 */
function cameronjonesweb_remove_custom_menu_item_class_from_li_element( $classes, $item, $args, $depth ) {
	$custom_classes = get_post_meta( $item->ID, '_menu_item_classes', true );
	$classes        = array_diff( $classes, $custom_classes );
	return $classes;
}