Walker_Category::start_el( string $output, object $category, int $depth, array $args = array(), int $id )

	public function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
		/** This filter is documented in wp-includes/category-template.php */
		$cat_name = apply_filters(
			'list_cats',
			esc_attr( $category->name ),
			$category
		);

		// Don't generate an element if the category name is empty.
		if ( ! $cat_name ) {
			return;
		}

		$link = '<a href="' . esc_url( get_term_link( $category ) ) . '" ';
		if ( $args['use_desc_for_title'] && ! empty( $category->description ) ) {
			/**
			 * Filters the category description for display.
			 *
			 * @since WP-1.2.0
			 *
			 * @param string $description Category description.
			 * @param object $category    Category object.
			 */
			$link .= 'title="' . esc_attr( strip_tags( apply_filters( 'category_description', $category->description, $category ) ) ) . '"';
		}

		$link .= '>';
		$link .= $cat_name . '</a>';

		if ( ! empty( $args['feed_image'] ) || ! empty( $args['feed'] ) ) {
			$link .= ' ';

			if ( empty( $args['feed_image'] ) ) {
				$link .= '(';
			}

			$link .= '<a href="' . esc_url( get_term_feed_link( $category->term_id, $category->taxonomy, $args['feed_type'] ) ) . '"';

			if ( empty( $args['feed'] ) ) {
				$alt = ' alt="' . sprintf(__( 'Feed for all posts filed under %s' ), $cat_name ) . '"';
			} else {
				$alt = ' alt="' . $args['feed'] . '"';
				$name = $args['feed'];
				$link .= empty( $args['title'] ) ? '' : $args['title'];
			}

			$link .= '>';

			if ( empty( $args['feed_image'] ) ) {
				$link .= $name;
			} else {
				$link .= "<img src='" . $args['feed_image'] . "'$alt" . ' />';
			}
			$link .= '</a>';

			if ( empty( $args['feed_image'] ) ) {
				$link .= ')';
			}
		}

		if ( ! empty( $args['show_count'] ) ) {
			$link .= ' (' . number_format_i18n( $category->count ) . ')';
		}
		if ( 'list' == $args['style'] ) {
			$output .= "\t<li";
			$css_classes = array(
				'cat-item',
				'cat-item-' . $category->term_id,
			);

			if ( ! empty( $args['current_category'] ) ) {
				// 'current_category' can be an array, so we use `get_terms()`.
				$_current_terms = get_terms( $category->taxonomy, array(
					'include' => $args['current_category'],
					'hide_empty' => false,
				) );

				foreach ( $_current_terms as $_current_term ) {
					if ( $category->term_id == $_current_term->term_id ) {
						$css_classes[] = 'current-cat';
					} elseif ( $category->term_id == $_current_term->parent ) {
						$css_classes[] = 'current-cat-parent';
					}
					while ( $_current_term->parent ) {
						if ( $category->term_id == $_current_term->parent ) {
							$css_classes[] =  'current-cat-ancestor';
							break;
						}
						$_current_term = get_term( $_current_term->parent, $category->taxonomy );
					}
				}
			}

			/**
			 * Filters the list of CSS classes to include with each category in the list.
			 *
			 * @since WP-4.2.0
			 *
			 * @see wp_list_categories()
			 *
			 * @param array  $css_classes An array of CSS classes to be applied to each list item.
			 * @param object $category    Category data object.
			 * @param int    $depth       Depth of page, used for padding.
			 * @param array  $args        An array of wp_list_categories() arguments.
			 */
			$css_classes = implode( ' ', apply_filters( 'category_css_class', $css_classes, $category, $depth, $args ) );

			$output .=  ' class="' . $css_classes . '"';
			$output .= ">$link\n";
		} elseif ( isset( $args['separator'] ) ) {
			$output .= "\t$link" . $args['separator'] . "\n";
		} else {
			$output .= "\t$link<br />\n";
		}
	}