Taken from http://www.ardamis.com/2011/06/02/fix-for-php-fatal-error-get_header-in-wordpress/

While making changes to my WordPress theme, I noticed that the error_log file in my theme folder contained dozens of PHP Fatal error lines:

[01-Jun-2011 14:25:15] PHP Fatal error:  Call to undefined function  get_header() in /home/accountname/public_html/ardamis.com/wp-content/themes/ars/index.php on line 7
[01-Jun-2011 20:58:23] PHP Fatal error:  Call to undefined function  get_header() in /home/accountname/public_html/ardamis.com/wp-content/themes/ars/index.php on line 7

The first seven lines of my theme’s index.php file:

<?php ini_set('display_errors', 0); ?>
<?php
	/*
	* @package WordPress
	* @subpackage Theme
	*/
	get_header();
?>

I realized that the error was being generated each time that my theme’s index.php file was called directly, and that the error was caused by the theme’s inability to locate the WordPress get_header function (which is completely normal). Thankfully, the descriptive error wasn’t being output to the browser, but was only being logged to the error_log file, due to the inclusion of the ini_set(‘display_errors’, 0); line. I had learned this the hard way a few months ago when I found that calling the theme’s index.php file directly would generate an error message, output to the browser, that would reveal my hosting account username as part of the absolute path to the file throwing the error.

I decided the best way to handle this would be to check to see if the file could find the get_header function, and if it could not, simply redirect the visitor to the site’s home page. The code I used to do this:

<?php ini_set('display_errors', 0); ?>
<?php
/**
* @package WordPress
* @subpackage Ars_Theme
*/
if (function_exists('get_header')) {
	get_header();
}else{
    /* Redirect browser */
    header("Location: http://www.ewea.org/SITENAME/");
    /* Make sure that code below does not get executed when we redirect. */
    exit;
}; ?>

See official documentation here:
http://codex.wordpress.org/Function_Reference/wp_list_pages

This here is the most interesting part (towards the bottom):

They can be styled with CSS selectors:

.pagenav { … } /* the outermost list item; contains whole list */
.page-item-2 { … } /* item for Page ID 2 */
.page_item { … } /* any Page item */
.current_page_item { … } /* the current Page */
.current_page_parent { … } /* parent of the current Page */
.current_page_ancestor { … } /* any ancestor of the current Page */

In order to achieve an accordion menu effect for instance, the following CSS can be used:

.pagenav  ul ul,
.pagenav .current_page_item ul ul,
.pagenav .current_page_ancestor ul ul,
.pagenav .current_page_ancestor .current_page_item ul ul,
.pagenav .current_page_ancestor .current_page_ancestor ul ul {
display: none;
}

.pagenav .current_page_item ul,
.pagenav .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_ancestor ul { 
display: block;
}