Wrapper function for wp_list_pages() that accepts more than one ID in exclude_tree and adds an include_tree parameter

slimline-wp-list-pages.php

<?php
/**
 * slimline_wp_list_pages function
 *
 * Builds include and exclude parameters from a tree before calling
 * a standard wp_list_pages call. All parameters are the same as the 
 * wp_list_pages functions with one exception listed below.
 *
 * @param string $args[ 'include_tree' ] Define a comma-separated list of Parent IDs to be included
 */
function slimline_wp_list_pages( $args ) {

	/**
	 * Set default includes and excludes since these are the only parameters
	 * we are editing with this function. Set echo since it will be used at the
	 * end of the function
	 */
	$includes_excludes = array(
		'echo'         => true,
		'exclude'      => '',
		'exclude_tree' => '',
		'include'      => '',
		'include_tree' => '',
	);

	// parse and merge args with defaults
	$args = wp_parse_args( $args, $includes_excludes );

	/**
	 * Get page IDs and children from include and exclude trees and append them to the standard include
	 * and exclude arguments
	 */
	$args[ 'exclude' ] = slimline_wp_list_pages_parse_tree( $args[ 'exclude'], $args[ 'exclude_tree' ] );
	$args[ 'include' ] = slimline_wp_list_pages_parse_tree( $args[ 'include'], $args[ 'include_tree' ] );

	/**
	 * Unset the include and exclude trees (we are done with them)
	 */
	unset( $args[ 'exclude_tree' ] );
	unset( $args[ 'include_tree' ] );

	/**
	 * Call wp_list_pages or return the value of wp_list_pages depending on the echo parameter
	 */
	if ( $args[ 'echo' ] )
		wp_list_pages( $args );
	else
		return wp_list_pages( $args );
}

/**
 * slimline_wp_list_pages_parse_tree function
 *
 * Helper function for slimline_wp_list_pages. Generates a comma-separated list for the include or exclude
 * parameters based on the given value of that parameter and the value of the related tree.
 *
 * @param string $list The comma-separated list of includes or excludes
 * @param string $tree The comma-separated list of include_tree or exclude_tree parent IDs
 * @return string $list The final comma-separated list of includes or excludes
 */
function slimline_wp_list_pages_parse_tree( $list = '', $tree = '' ) {

	if ( ! $tree )
		return $list; // return default or given list value if no tree to parse

	$list = explode( ',', $list ); // turn $list into an array so we can easily add to it later
	$tree_array = $tree = explode( ',', $tree ); // turn $tree into an array so we can iterate through it

	/**
	 * Iterate through the $tree array to get the children of each page in the $tree. Add the IDs of each
	 * child into $tree_array to merge into the $list
	 */
	foreach ( $tree as $parent ) {

		if ( ! ( $child_pages = get_pages( array( 'child_of' => $parent ) ) ) )
			continue; // skip processing if no child pages found for the current parent

		foreach ( $child_pages as $child_page )
			$tree_array[] = $child_page->ID;

	}

	$list = array_merge( $list, $tree_array );

	return implode( ',', $list );
}

Originally posted at http://www.michaeldozark.com/tips/even-better-includes-and-excludes-for-wp_list_pages/