Source

File: wp-includes/category-template.php

function get_the_category_list( $separator = '', $parents = '', $post_id = false ) {
	global $wp_rewrite;

	if ( ! is_object_in_taxonomy( get_post_type( $post_id ), 'category' ) ) {
		/** This filter is documented in wp-includes/category-template.php */
		return apply_filters( 'the_category', '', $separator, $parents );
	}

	/**
	 * Filters the categories before building the category list.
	 *
	 * @since 4.4.0
	 *
	 * @param WP_Term[] $categories An array of the post's categories.
	 * @param int|bool  $post_id    ID of the post we're retrieving categories for.
	 *                              When `false`, we assume the current post in the loop.
	 */
	$categories = apply_filters( 'the_category_list', get_the_category( $post_id ), $post_id );

	if ( empty( $categories ) ) {
		/** This filter is documented in wp-includes/category-template.php */
		return apply_filters( 'the_category', __( 'Uncategorized' ), $separator, $parents );
	}

	$rel = ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) ? 'rel="category tag"' : 'rel="category"';

	$thelist = '';
	if ( '' === $separator ) {
		$thelist .= '<ul class="post-categories">';
		foreach ( $categories as $category ) {
			$thelist .= "\n\t<li>";
			switch ( strtolower( $parents ) ) {
				case 'multiple':
					if ( $category->parent ) {
						$thelist .= get_category_parents( $category->parent, true, $separator );
					}
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '" ' . $rel . '>' . $category->name . '</a></li>';
					break;
				case 'single':
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '"  ' . $rel . '>';
					if ( $category->parent ) {
						$thelist .= get_category_parents( $category->parent, false, $separator );
					}
					$thelist .= $category->name . '</a></li>';
					break;
				case '':
				default:
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '" ' . $rel . '>' . $category->name . '</a></li>';
			}
		}
		$thelist .= '</ul>';
	} else {
		$i = 0;
		foreach ( $categories as $category ) {
			if ( 0 < $i ) {
				$thelist .= $separator;
			}
			switch ( strtolower( $parents ) ) {
				case 'multiple':
					if ( $category->parent ) {
						$thelist .= get_category_parents( $category->parent, true, $separator );
					}
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '" ' . $rel . '>' . $category->name . '</a>';
					break;
				case 'single':
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '" ' . $rel . '>';
					if ( $category->parent ) {
						$thelist .= get_category_parents( $category->parent, false, $separator );
					}
					$thelist .= "$category->name</a>";
					break;
				case '':
				default:
					$thelist .= '<a href="' . esc_url( get_category_link( $category->term_id ) ) . '" ' . $rel . '>' . $category->name . '</a>';
			}
			++$i;
		}
	}

	/**
	 * Filters the category or list of categories.
	 *
	 * @since 1.2.0
	 *
	 * @param string $thelist   List of categories for the current post.
	 * @param string $separator Separator used between the categories.
	 * @param string $parents   How to display the category parents. Accepts 'multiple',
	 *                          'single', or empty.
	 */
	return apply_filters( 'the_category', $thelist, $separator, $parents );
}