Generate a year -> month based list of posts in WordPress, similar to the native Archives widget, only with months being children of years as oposed to a flat list

year-month-archive-list.php

<?php // In functions.php:

/* -- Year and month based archive listings -- */ 
function posts_by_year() {
    // array to use for results
    $years = array();
    // get posts from WP
    $posts = get_posts(array(
        'numberposts' => -1,
        'orderby' => 'post_date',
        'order' => 'ASC',
        'post_type' => 'national-press',
        'post_status' => 'publish'
    ));
    // loop through posts, populating $years arrays
    foreach($posts as $post) {
        $years[date('Y', strtotime($post->post_date))][date('F', strtotime($post->post_date))] = $post;
    }
    
    // reverse sort by year
    krsort($years);
    
    return $years;
}
?>


<?php // and in your theme file:
// All of the extra classes on <li>'s are to mimic the output of WP's native menus
foreach(posts_by_year() as $year => $posts) : ?>
    <li id="" class="main-menu-item menu-item menu-item-has-children <?php echo ((int)$get_year == (int)$year ? 'current-page-ancestor current-menu-ancestor current-menu-parent current-page-parent current_page_parent current_page_ancestor current-menu-item' : ''); ?>">
        <a href="<?php echo site_url('/news-press/national-press/') . $year . "/"; ?>" class="toggle-link menu-link main-menu-link"><?php echo $year; ?></a>
        <ul class="sub-menu">
            <?php foreach($posts as $key => $val) : ?>
                <?php $is_current_page = is_current_page(site_url('/news-press/national-press/') . $year . "/" . $key . "/"); // is_current_page() function also in Gist repo :) ?>
                <li id="" class="sub-menu-item menu-item <?php echo $is_current_page ? "current-menu-item" : ""; ?>">
                    <a href="<?php echo site_url('/news-press/national-press/') . $year . "/" . $key . "/"; ?>" class="menu-link sub-menu-link"><?php echo $key; ?></a>
                </li>
            <?php endforeach; ?>
        </ul>
    </li>
<?php endforeach; ?>