How to group Wordpress posts in years and then months?

The following solution has been fully tested and works seamlessly fine for the default wordpress post type. If you want to run it for your custome post type that has its own custom fields then you may encounter some discrepancies. Therefore, feel free to customize it for your own custom post type as needed.

As we talked on this question, when you use get_posts it will return post objects. Each post object contains the following properties/data:

WP_Post Object(    [ID] =>    [post_author] =>    [post_date] =>     [post_date_gmt] =>     [post_content] =>     [post_title] =>     [post_excerpt] =>     [post_status] =>    [comment_status] =>    [ping_status] =>     [post_password] =>     [post_name] =>    [to_ping] =>     [pinged] =>     [post_modified] =>     [post_modified_gmt] =>    [post_content_filtered] =>     [post_parent] =>     [guid] =>     [menu_order] =>    [post_type] =>    [post_mime_type] =>     [comment_count] =>    [filter] =>)

For example if you need, title, content, author, exerpt of the post, then you could create a multi-dimensional array and use array_push function to create your custom array like so:

$posts = get_posts(array(  'post_type' => 'history', // Since 'history' is a custom post type, I tested it on 'post'//'post_type' => 'post'  'posts_per_page' => -1,  'orderby'   => 'date'));// You could see the number of posts for your queryecho "Number of posts found: ". count($posts) . "<br>";$your_custom_array_yearly = array();foreach ($posts as $post) {  setup_postdata($post);  $time = strtotime($post->post_date);  $year = date('Y', $time);  $mon = date('F', $time);  if (!($your_custom_array_yearly[$year][$mon])) {    $your_custom_array_yearly[$year][$mon] = array();    array_push(      $your_custom_array_yearly[$year][$mon],      [        'title'   => $post->post_title,        'excerpt' => $post->post_excerpt,        'author'  => $post->post_author,        'content' => $post->post_content,      ]    );  } else {    array_push(      $your_custom_array_yearly[$year][$mon],      [        'title'   => $post->post_title,        'excerpt' => $post->post_excerpt,        'author'  => $post->post_author,        'content' => $post->post_content,      ]    );  };}wp_reset_postdata();

For debugging/investigating the array, you could use this:

echo "<pre>";print_r($your_custom_array_yearly);echo "</pre>";

Now that you have your own custom array, then you could loop through it and output your data:

foreach ((array)$your_custom_array_yearly as $yearly => $yvalue) {  echo $yearly . ": <br>";  echo "<ul>";  foreach ($yvalue as $monthly => $mvalue) {    echo "<li>" . $monthly . ": </li><ul>";    foreach ($mvalue as $monthly_k => $monthly_v) {      foreach ($monthly_v as $post_title_k => $post_title_v) {        echo "<li>" . $post_title_k . ": " . $post_title_v . "</li></ul>";      }    }  }  echo "</ul>";}

The above code will output the following result:

2021:  January:    1.Title: Post Title    1.Excerpt: Post excerpt    1.Author: Post author    1.Content: Post content    2.Title: Post Title    2.Excerpt: Post excerpt    2.Author: Post author    2.Content: Post content  March:    3.Title: Post Title    3.Excerpt: Post excerpt    3.Author: Post author    3.Content: Post content2020:  May:    4.Title: Post Title    4.Excerpt: Post excerpt    4.Author: Post author    4.Content: Post content

Create an multidimensional array like this way and print it. You might get appropriate result.

$master = [];foreach ( $posts as $post ) {    setup_postdata( $post );    $time = strtotime( $post->post_date );    $year = date( 'Y', $time );    $mon = date( 'F', $time );        $master[$year][$mon][] = $post;}wp_reset_postdata();echo '<pre>';print_r($master);echo '</pre>';