How do I list categories and the common categories for posts beneath those categories?

Question

I have four categories:

• 2013
• 2012
• 2011
• 2010

These categories contain posts that are also categorized

• Cat A
• Cat B
• Cat C

So basically there's a bunch of posts that are categorized (2013, Cat A), (2013, Cat B), (2012, Cat A), (2012, Cat C), etc

Without having to add Cat A, Cat B and Cat C as four sets of child categories under 2013, 2012, 2011 and 2010 is there a way that I can programmatically list those unique subcats with posts under each year so that they list as:

• 2013
• Cat A
• Cat B
• Cat C

• 2012
• Cat A
• Cat B
• Cat C

• 2011
• Cat A
• Cat B
• Cat C

• 2010
• Cat A
• Cat B
• Cat C

AND have those subcats link to their respective category page?

Answer 1

Untested, but I think this will do the trick!

<?php

// Get date Categories
$date_cats = get_categories(array(
    'include' => '1,2,3,4'
));

// Loop through date categories
foreach($date_cats as $date_cat) :

    // Get link of current date category
    $date_cat_link = get_category_link( $date_cat->cat_ID ); ?>

    <ul>
        <li><a href="<?php echo $date_cat_link; ?>"><?php echo $date_cat->name; ?></a>
            <ul>
            <?php
            // Get ID of current date category
            $date_cat_id = $date_cat->cat_ID;

            // Get ABC categories
            $abc_cats = get_categories(array(
                'include' => '5,6,7'
            ));

            // Loop through ABC catagories
            foreach($abc_cats as $abc_cat) :

                // Get ID of current ABC category
                $abc_cat_id = $abc_cat->cat_ID;
                // Get link of current ABC category
                $abc_cat_link = get_category_link( $abc_cat->cat_ID );

                // Run a query for posts that are in this date category AND this ABC category
                $query = new WP_Query( array(
                    'posts_per_page' => -1,
                    'category__and' => array( $date_cat_id, $abc_cat_id )
                ));

                // If there are posts, show this ABC category's name and link
                if ( $query->have_posts() ) { ?>

                <li><a href="<?php echo $abc_cat_link; ?>"><?php echo $abc_cat->name; ?></a></li>

                <?php } ?>

            <?php endforeach; ?>
            </ul>
        </li>
    </ul>

<?php endforeach; ?>