With this solution you can, for example, in dependence of a page, load a specific layout. This small request I'll take as an example and show a function that gives queries to pages / sub-pages and I will show an example in action.

Background

If you create in WordPress a new page, then you can define it in the attributes as a subpage of an existing page. This can not only create tree views, but also structures which are not uncommonly needed in a CMS.

If you want control the output depending on the page, then you use the Conditional Tag is_page(), which you use to implement the requirement. For not using the tag for every page and use the template constantly, you can use a simple function, which queries the main page and returns a true back (true) when there is a subpage of the main page.

Solution

This function, the following syntax is used in the functions.php of the theme.

// check ID post and child-post
function has_parent( $post, $post_id ) {
	if ($post->ID == $post_id)
		return true;
	elseif ($post->post_parent == 0)
		return false;
	else
		return has_parent( get_post($post->post_parent), $post_id );
}

This little function checks whether the output page is a page or subpage of a page with the passed ID; sounds complicated, but it really isn't. This feature can be, after put in the theme, be used in all the templates now. The following example checks whether it is a page or subpage of the ID 1, and if so, then the stylesheet Alternative 1 (alternative_1.css) will be output.

Otherwise will be ID 2, or if it doesn't fit to none of the queries, then the default stylesheet will be used.

To have the ability to expand a little, I set in the query variable $bodyclass that I output in the body-tag. Thus I have an additional point and depending on the page to realize an according design.

    <?php
    $bodyclass = '';
    if ( function_exists('has_parent') ) {
	    if ( has_parent($wp_query->post, 1) ) {
	        echo '<link rel="stylesheet" href="' . get_bloginfo('stylesheet_directory') . '/alternative_1.css" type="text/css" media="screen" />' . "\n";
	        $bodyclass = 'class="alternative_1"';
	    } elseif ( has_parent($wp_query->post, 2) ) {
	        echo '<link rel="stylesheet" href="' . get_bloginfo('stylesheet_directory') . '/alternative_2.css" type="text/css" media="screen" />' . "\n";
	        $bodyclass = 'class="alternative_2"';
	    } else {
	    	echo '<link rel="stylesheet" href="' . get_bloginfo('stylesheet_directory') . '/default.css" type="text/css" media="screen" />' . "\n";
	    }
  	} ?>

</head>
<body<?php echo ' ' . $bodyclass; ?>>

Conclusion

I think that this small possibility can have a large effect in the output and can have various possibilities. Also this solution can be realized with the change of a theme, which I mentioned in the article „Change your WordPress Theme on Dependency“.

Thank you for reading and even more I thank you for contributing any improvements in the comments section.

Following solution only works with WordPress 2.5 or higher. If you need help for a lower version, let me know in the comment field and I'll try to help.

Number of posts

$num_posts = wp_count_posts( 'post' );
$num_posts = $num_posts->publish; //publish, draft

Number of pages

$num_pages = wp_count_posts( 'page' );
$num_pages = $num_pages->publish; //publish

Number of categories

$num_cats  = wp_count_terms('category');

Number of Tags

$num_tags  = wp_count_terms('post_tag');

Number of comments

$num_comm  = get_comment_count();
$num_comm  = $num_comm['approved']; //approved, awaiting_moderation, spam, tot

// Solution 2
$num_comm2 = wp_count_comments( );
$num_comm2 = $num_comm2->approved; //approved, moderated, spam, total_comments

Output

The above syntax shows simple how to get the values without including HTML. In another example I show you all results in HTML as a list. Everybody can adjust the format to integrate in their design.

<?php
$num_posts = wp_count_posts( 'post' );
$num_posts = $num_posts->publish; //publish, draft
$num_posts = sprintf( __ngettext( '%s Post', '%s Posts', $num_posts ), number_format_i18n( $num_posts ) );

$num_pages = wp_count_posts( 'page' );
$num_pages = $num_pages->publish; //publish
$num_pages = sprintf( __ngettext( '%s Page', '%s Pages', $num_pages ), number_format_i18n( $num_pages ) );

$num_cats  = wp_count_terms('category');
$num_tags  = wp_count_terms('post_tag');

$num_comm  = get_comment_count();
$num_comm  = $num_comm['approved']; //approved, awaiting_moderation, spam, total_comments
$num_comm  = sprintf( __ngettext( '%s Categorie', '%s Categories', $num_comm ), number_format_i18n( $num_comm ) );
$num_comm2 = wp_count_comments( );
$num_comm2 = $num_comm2->approved; //approved, moderated, spam, total_comments

echo '<ul>';
echo '<li>Posts: ' . $num_posts . '</li>';
echo '<li>Pages: ' . $num_pages . '</li>';
echo '<li>Categories: ' . $num_cats . '</li>';
echo '<li>Tags: ' . $num_tags . '</li>';
echo '<li>Comments: ' . $num_comm . '</li>';
echo '<li>Comments 2: ' . $num_comm2 . '</li>';
echo '</ul>';
?>

You can adjust the design of these lists. You can use classes or IDs. An example is in article „Total Amount of Comments“ (in german language), where you can see a button with the amount of comments.