Override the output of the WooCommerce product categories widget to be a Bootstrap accordion

widget-product-categories.php

<?php
/**
* Product Categories Widget
* Modifies the WooCommerce product categories widget to display as a Bootstrap accordion.
*
* @package WooCommerce/Widgets
* @version 2.3.0
*/

defined( 'ABSPATH' ) || exit;

/**
 * Product categories widget class.
 *
 * @extends WC_Widget
 */
class DE_Widget_Product_Categories extends WC_Widget {

	/**
	 * Category ancestors.
	 *
	 * @var array
	 */
	public $cat_ancestors;

	/**
	 * Current Category.
	 *
	 * @var bool
	 */
	public $current_cat;

	/**
	 * Constructor.
	 */
	public function __construct() {
		$this->widget_cssclass    = 'woocommerce widget_product_categories';
		$this->widget_description = __( 'An accordion list of product categories and subcategories.', 'woocommerce' );
		$this->widget_id          = 'woocommerce_product_categories--accordion';
		$this->widget_name        = __( 'Product Categories (Accordion)', 'woocommerce' );
		$this->settings           = array(
			'title'        => array(
				'type'  => 'text',
				'std'   => __( 'Product Categories', 'woocommerce' ),
				'label' => __( 'Title (not shown visually)', 'woocommerce' ),
			),
			'orderby'            => array(
				'type'    => 'select',
				'std'     => 'name',
				'label'   => __( 'Order by', 'woocommerce' ),
				'options' => array(
					'order' => __( 'Category order', 'woocommerce' ),
					'name'  => __( 'Name', 'woocommerce' ),
				),
			),
			'count'              => array(
				'type'  => 'checkbox',
				'std'   => 0,
				'label' => __( 'Show product counts', 'woocommerce' ),
			),
			'hide_empty'         => array(
				'type'  => 'checkbox',
				'std'   => 0,
				'label' => __( 'Hide empty categories', 'woocommerce' ),
			),
		);

		parent::__construct();
	}

	/**
	 * Output widget.
	 *
	 * @see WP_Widget
	 * @param array $args     Widget arguments.
	 * @param array $instance Widget instance.
	 */
	public function widget( $args, $instance ) {
		global $wp_query, $post;

		$count              = isset( $instance['count'] ) ? $instance['count'] : $this->settings['count']['std'];
		$hierarchical       = isset( $instance['hierarchical'] ) ? $instance['hierarchical'] : $this->settings['hierarchical']['std'];
		$show_children_only = isset( $instance['show_children_only'] ) ? $instance['show_children_only'] : $this->settings['show_children_only']['std'];
		$dropdown           = isset( $instance['dropdown'] ) ? $instance['dropdown'] : $this->settings['dropdown']['std'];
		$orderby            = isset( $instance['orderby'] ) ? $instance['orderby'] : $this->settings['orderby']['std'];
		$hide_empty         = isset( $instance['hide_empty'] ) ? $instance['hide_empty'] : $this->settings['hide_empty']['std'];
		$dropdown_args      = array(
			'hide_empty' => $hide_empty,
		);
		$list_args          = array(
			'show_count'   => $count,
			'hierarchical' => $hierarchical,
			'taxonomy'     => 'product_cat',
			'hide_empty'   => $hide_empty,
		);
		$max_depth          = absint( isset( $instance['max_depth'] ) ? $instance['max_depth'] : $this->settings['max_depth']['std'] );

		$list_args['menu_order'] = false;
		$dropdown_args['depth']  = $max_depth;
		$list_args['depth']      = $max_depth;

		if ( 'order' === $orderby ) {
			$list_args['orderby']      = 'meta_value_num';
			$dropdown_args['orderby']  = 'meta_value_num';
			$list_args['meta_key']     = 'order';
			$dropdown_args['meta_key'] = 'order';
		}

		$this->current_cat   = false;
		$this->cat_ancestors = array();

		if ( is_tax( 'product_cat' ) ) {
			$this->current_cat   = $wp_query->queried_object;
			$this->cat_ancestors = get_ancestors( $this->current_cat->term_id, 'product_cat' );

		} elseif ( is_singular( 'product' ) ) {
			$terms = wc_get_product_terms(
				$post->ID,
				'product_cat',
				apply_filters(
					'woocommerce_product_categories_widget_product_terms_args',
					array(
						'orderby' => 'parent',
						'order'   => 'DESC',
					)
				)
			);

			if ( $terms ) {
				$main_term           = apply_filters( 'woocommerce_product_categories_widget_main_term', $terms[0], $terms );
				$this->current_cat   = $main_term;
				$this->cat_ancestors = get_ancestors( $main_term->term_id, 'product_cat' );
			}
		}

		// Show Siblings and Children Only.
		if ( $show_children_only && $this->current_cat ) {
			if ( $hierarchical ) {
				$include = array_merge(
					$this->cat_ancestors,
					array( $this->current_cat->term_id ),
					get_terms(
						'product_cat',
						array(
							'fields'       => 'ids',
							'parent'       => 0,
							'hierarchical' => true,
							'hide_empty'   => false,
						)
					),
					get_terms(
						'product_cat',
						array(
							'fields'       => 'ids',
							'parent'       => $this->current_cat->term_id,
							'hierarchical' => true,
							'hide_empty'   => false,
						)
					)
				);
				// Gather siblings of ancestors.
				if ( $this->cat_ancestors ) {
					foreach ( $this->cat_ancestors as $ancestor ) {
						$include = array_merge(
							$include,
							get_terms(
								'product_cat',
								array(
									'fields'       => 'ids',
									'parent'       => $ancestor,
									'hierarchical' => false,
									'hide_empty'   => false,
								)
							)
						);
					}
				}
			} else {
				// Direct children.
				$include = get_terms(
					'product_cat',
					array(
						'fields'       => 'ids',
						'parent'       => $this->current_cat->term_id,
						'hierarchical' => true,
						'hide_empty'   => false,
					)
				);
			}

			$list_args['include']     = implode( ',', $include );
			$dropdown_args['include'] = $list_args['include'];

			if ( empty( $include ) ) {
				return;
			}
		} elseif ( $show_children_only ) {
			$dropdown_args['depth']        = 1;
			$dropdown_args['child_of']     = 0;
			$dropdown_args['hierarchical'] = 1;
			$list_args['depth']            = 1;
			$list_args['child_of']         = 0;
			$list_args['hierarchical']     = 1;
		}

		$this->widget_start( $args, $instance );


		include_once WC()->plugin_path() . '/includes/walkers/class-wc-product-cat-list-walker.php';

		$list_args['walker']                     = new WC_Product_Cat_List_Walker();
		$list_args['title_li']                   = '';
		$list_args['pad_counts']                 = 1;
		$list_args['show_option_none']           = __( 'No product categories exist.', 'woocommerce' );
		$list_args['current_category']           = ( $this->current_cat ) ? $this->current_cat->term_id : '';
		$list_args['current_category_ancestors'] = $this->cat_ancestors;

		// Check which category we're currently viewing
		$queried_object = get_queried_object();
		$current_cat_id = 0;
		if(isset($queried_object->term_id)) {
			$current_cat_id = $queried_object->term_id;
		}

		// Get the top level categories
		$top_level_categories = get_terms(array(
			'taxonomy' => 'product_cat',
			'hide_empty' => $list_args['hide_empty'],
			'parent' => 0,
			'exclude' => array(15), // exclude uncategorised
		));

		// Check if the caregory we're on has ancestors, and get the top-level one
		$ancestors = get_ancestors($current_cat_id, 'product_cat'); // Get a list of ancestors
		$ancestors = array_reverse($ancestors); //Reverse the array to put the top level ancestor first
		$top_level_ancestor = 0;
		if($ancestors) {
			$top_level_ancestor = $ancestors[0];
		}

		// Start widget HTML output
		echo '<ul class="product-categories">';
			foreach($top_level_categories as $category) {

				$cat_id = 'collapse-' . $category->slug;
				$count = '';
				if($list_args['pad_counts']) {
					$count = '<span class="count">('.$category->count.')</span>';
				}

				$subcategories = get_terms(array(
					'taxonomy' => 'product_cat',
					'hide_empty' => $list_args['hide_empty'],
					'parent' => $category->term_id,
				));

				if($subcategories) {
					// If this is the current category or a child of it, set up to open by default and be bold
					$aria_expanded = 'false';
					$current_class = '';
					$panel_class = '';
					$button_class = '';
					if(($category->term_id == $current_cat_id) || ($category->term_id == $top_level_ancestor)) {
						$aria_expanded = 'true';
						$current_class = 'current';
						$panel_class = 'show';
						$button_class = 'open';
					}

					echo '<li class="product-categories__category">';

						echo '<span class="widget__title">';

							echo '<a href="'.get_term_link($category->term_id, 'product_cat').'">';
								echo '<h3 class="widget__title__heading '.$current_class.'">' . ucfirst($category->name) . $count . '</h3>';
							echo '</a>';

							echo '<button class="widget__collapse-button '.$button_class.'" data-toggle="collapse" data-target="#'.$cat_id.'" aria-expanded="'.$aria_expanded.'" aria-controls="'.$cat_id.'">';
								echo '<i class="fa far fa-chevron-down"></i>';
							echo '</button>';

						echo '</span>';

						echo '<ul id="'.$cat_id.'" class="collapse product-categories__children '.$panel_class.'">';

							foreach ($subcategories as $subcategory) {
								$url = get_term_link($subcategory->term_id, 'product_cat');
								$sub_count = '';
								$sub_current_class = '';
								$current_term = get_term($current_cat_id);
								if(isset($current_term->term_id)) { // checks if we're on a category page, to prevent errors on the shop page
									if (($subcategory->term_id == $current_cat_id) || ($subcategory->term_id == $current_term->parent)) {
										$sub_current_class = 'current';
									}
								}
								if($list_args['pad_counts']) {
									$sub_count = '<span class="count">('.$subcategory->count.')</span>';
								}

								echo '<li class="product-categories__children__subcategory">';
									echo '<a href="'.$url.'" class="'.$sub_current_class.'">'.ucfirst($subcategory->name) . $sub_count . '</a>';
								echo '<li>';

								// Third level categories
								$third_level_categories = get_terms(array(
									'taxonomy' => 'product_cat',
									'hide_empty' => $list_args['hide_empty'],
									'parent' => $subcategory->term_id,
								));

								if($third_level_categories) {
									echo '<ul class="product-categories__grandchildren">';
										foreach($third_level_categories as $third_level_category) {
											$third_current_class = '';
											if($third_level_category->term_id == $current_cat_id) {
												$third_current_class = 'current';
											}
											$url = get_term_link($third_level_category->term_id, 'product_cat');

											echo '<li class="product-categories__grandchildren__subcategory">';
												echo '<a href="'.$url.'" class="'.$third_current_class.'">'.ucfirst($third_level_category->name).'</a>';
											echo '<li>';
										}
									echo '</ul>';
								}

							}
						echo '</ul>';
					echo '</li>';
				}
				// No subcategories
				else {
					$url = get_term_link($category->term_id);
					echo '<li class="product-categories__category">';
						echo '<a class="widget__collapse-button" href="'.$url.'">';
						echo '<h3 class="widget__title">' . ucfirst($category->name) . $count . '</h3>';
						echo '</a>';
					echo '</li>';
				}
			}
		echo '</ul>';
		// End widget HTML output


		$this->widget_end( $args );
	}
}

widgets.php

<?php
/**
 * Register custom widgets
 */
function doublee_register_widgets() {

	if(is_woocommerce_active()) {
		require 'woocommerce/widget-product-categories.php';
		register_widget('DE_Widget_Product_Categories');
	}
}
add_action('widgets_init', 'doublee_register_widgets');