wp_list_pages() WordPress Function

function wp_list_pages( $args = '' ) {
	$defaults = array(
		'depth'        => 0,
		'show_date'    => '',
		'date_format'  => get_option( 'date_format' ),
		'child_of'     => 0,
		'exclude'      => '',
		'title_li'     => __( 'Pages' ),
		'echo'         => 1,
		'authors'      => '',
		'sort_column'  => 'menu_order, post_title',
		'link_before'  => '',
		'link_after'   => '',
		'item_spacing' => 'preserve',
		'walker'       => '',
	);

	$parsed_args = wp_parse_args( $args, $defaults );

	if ( ! in_array( $parsed_args['item_spacing'], array( 'preserve', 'discard' ), true ) ) {
		// Invalid value, fall back to default.
		$parsed_args['item_spacing'] = $defaults['item_spacing'];
	}

	$output       = '';
	$current_page = 0;

	// Sanitize, mostly to keep spaces out.
	$parsed_args['exclude'] = preg_replace( '/[^0-9,]/', '', $parsed_args['exclude'] );

	// Allow plugins to filter an array of excluded pages (but don't put a nullstring into the array).
	$exclude_array = ( $parsed_args['exclude'] ) ? explode( ',', $parsed_args['exclude'] ) : array();

	/**
	 * Filters the array of pages to exclude from the pages list.
	 *
	 * @since 2.1.0
	 *
	 * @param string[] $exclude_array An array of page IDs to exclude.
	 */
	$parsed_args['exclude'] = implode( ',', apply_filters( 'wp_list_pages_excludes', $exclude_array ) );

	$parsed_args['hierarchical'] = 0;

	// Query pages.
	$pages = get_pages( $parsed_args );

	if ( ! empty( $pages ) ) {
		if ( $parsed_args['title_li'] ) {
			$output .= '<li class="pagenav">' . $parsed_args['title_li'] . '<ul>';
		}
		global $wp_query;
		if ( is_page() || is_attachment() || $wp_query->is_posts_page ) {
			$current_page = get_queried_object_id();
		} elseif ( is_singular() ) {
			$queried_object = get_queried_object();
			if ( is_post_type_hierarchical( $queried_object->post_type ) ) {
				$current_page = $queried_object->ID;
			}
		}

		$output .= walk_page_tree( $pages, $parsed_args['depth'], $current_page, $parsed_args );

		if ( $parsed_args['title_li'] ) {
			$output .= '</ul></li>';
		}
	}

	/**
	 * Filters the HTML output of the pages to list.
	 *
	 * @since 1.5.1
	 * @since 4.4.0 `$pages` added as arguments.
	 *
	 * @see wp_list_pages()
	 *
	 * @param string    $output      HTML output of the pages list.
	 * @param array     $parsed_args An array of page-listing arguments. See wp_list_pages()
	 *                               for information on accepted arguments.
	 * @param WP_Post[] $pages       Array of the page objects.
	 */
	$html = apply_filters( 'wp_list_pages', $output, $parsed_args, $pages );

	if ( $parsed_args['echo'] ) {
		echo $html;
	} else {
		return $html;
	}
}