get_the_category_rss

 *
 * @param string $type Optional, default is the type returned by get_default_feed().
 * @return string All of the post categories for displaying in the feed.
 */
function get_the_category_rss( $type = null ) {
	if ( empty( $type ) ) {
		$type = get_default_feed();
	}
	$categories = get_the_category();
	$tags       = get_the_tags();
	$the_list   = '';
	$cat_names  = array();

	$filter = 'rss';
	if ( 'atom' === $type ) {
		$filter = 'raw';
	}

	if ( ! empty( $categories ) ) {
		foreach ( (array) $categories as $category ) {
			$cat_names[] = sanitize_term_field( 'name', $category->name, $category->term_id, 'category', $filter );
		}
	}

	if ( ! empty( $tags ) ) {
		foreach ( (array) $tags as $tag ) {
			$cat_names[] = sanitize_term_field( 'name', $tag->name, $tag->term_id, 'post_tag', $filter );
		}
	}

	$cat_names = array_unique( $cat_names );

	foreach ( $cat_names as $cat_name ) {
		if ( 'rdf' === $type ) {
			$the_list .= "\t\t<dc:subject><![CDATA[$cat_name]]></dc:subject>\n";
		} elseif ( 'atom' === $type ) {
			$the_list .= sprintf( '<category scheme="%1$s" term="%2$s" />', esc_attr( get_bloginfo_rss( 'url' ) ), esc_attr( $cat_name ) );
		} else {
			$the_list .= "\t\t<category><![CDATA[" . html_entity_decode( $cat_name, ENT_COMPAT, get_option( 'blog_charset' ) ) . "]]></category>\n";
		}
	}

	/**
	 * Filters all of the post categories for display in a feed.
	 *
	 * @since 1.2.0
	 *
	 * @param string $the_list All of the RSS post categories.
	 * @param string $type     Type of feed. Possible values include 'rss2', 'atom'.