Custom WordPress Walker for Michael McNeill

functions.php

<?php

/**
 * Custom Menu Walker
 *
 * Author: Chris Mewhort
 * Author URI: http://chrismewhort.com
 * Walker URI: https://codeable.io/developers/chris-mewhort
 *
 * Reference Markup URI: https://gist.github.com/michaelryanmcneill/381ed1a28726fe0eacb0973a7de40b92
 */
class Codeable_Walker_Menu extends Walker_Nav_Menu {
	// Tell Walker where to inherit it's parent and id values
	var $db_fields = array(
			'parent' => 'menu_item_parent', 
			'id'     => 'db_id'
	);

	function start_lvl( &$output, $depth = 0, $args = array() ) {
	}

	function end_lvl( &$output, $depth = 0, $args = array() ) {
	}

	/**
	 * At the start of each element
	 *
	 */
	function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		if ($depth === 0) {
			$output .= sprintf( "\n
				<div class=\"%s\">
					<div class=\"%s\">
						<a class=\"accordion-toggle\" data-toggle=\"collapse\" data-parent=\"%s\" href=\"%s\">
							<li>%s</li>
						</a>
					</div>",
				'accordion-group',
				'accordion-heading',
				'#'.$args->menu_id,
				$item->url,
				$item->title,
				$item->post_name,
				$class_for_item
			);
		}
		elseif ($depth === 1) {
			// Setup accordion body classes
			$accordion_body_classes = 'accordion-body collapse';
			$accordion_body_classes .= ($item->current) ? ' in':'';

			// Get parent menu item
			$parent_item_id = get_post_meta( $item->menu_item_parent, '_menu_item_object_id', true );
			$parent_item = get_category( $parent_item_id );

			// Manage Pages & Categories
			if ($parent_item->slug !== 'customwalker') {
				$parent_slug = $parent_item->slug;
			}
			else {
				$parent_item = get_post( $parent_item_id );
				$parent_slug = $parent_item->post_name;
			}

			$output .= sprintf( "\n
					<div id=\"%s\" class=\"%s\">
						<div class=\"%s\">
							<li>
								<a href=\"%s\">
									%s
								</a>
							</li>
						</div>",
				$parent_slug,
				$accordion_body_classes,
				'accordion-inner',
				$item->url,
				$item->title
			);
		}
	}

	function end_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		if ($depth === 0) {
			$output .= sprintf( "
				</div>\n");
		}
		elseif ($depth === 1) {
			$output .= sprintf( "
					</div>");
		}
	}
}

header.php

<?php

$nav_args = array(
  'container' => 'div',
  'container_class' => 'side-nav-collapse',
  'menu' => 2, //menu id
  'items_wrap' => '<ul><div id="%1$s" class="%2$s">%3$s</div></ul>',
  'menu_id' => 'accordion2',
  'menu_class' => 'accordion',
  'walker' => new Codeable_Walker_Menu() //use our custom walker
);
wp_nav_menu( $nav_args );