How can you access category information from a theme?

Question

I am making a wordpress site to host my artwork and comics, each comic series is in its own category, so I would like to display the category information in the site through my theme/a plugin I make.

There are three things I would like to do:

• Show the category Title, Description and a url to it's archive by using it's category ID.
• Get the Ids of the oldest and newest post in that category so I can add links to "Start Reading" and "Read the latest"
• Sort an array of category ids by the date of the latest post, so the most recently updated series can automatically float to the top of the list.

Is it possible to do this?

Answer 1

Show the category title, description and a url to it's archive by using it's category ID:

• Get category title: get_the_category_by_ID()
• Get category description (ID optional): category_description()
• Get category archive URL: get_category_link()

Get the Ids of the oldest and newest post in that category:

// Get oldest post ID
$posts = get_posts( array(
    'category' => 1,
    'orderby' => 'date',
    'order' => 'ASC',
    'post_type' => 'post',
    'post_status' => 'publish', 
) );
$oldest_post_id = $posts[0]->ID;

// Get newest post ID
$posts = get_posts( array(
    'category' => 1,
    'orderby' => 'date',
    'order' => 'DESC',
    'post_type' => 'post',
    'post_status' => 'publish', 
) );
$newest_post_id = $posts[0]->ID;

Sort an array of category ids by the date of the latest post:

function get_categories_by_the_date() {
    $categories_by_the_date = array();
    $categories = get_categories();

    foreach ( $categories as $category ) {
        $posts = get_posts( array(
            'category' => $category->term_id,
            'orderby' => 'date',
            'order' => 'DESC',
            'post_type' => 'post',
            'post_status' => 'publish', 
        ) );
        $categories_by_the_date[strtotime($posts[0]->post_date)] = $category->term_id;
    }

    krsort($categories_by_the_date);

    return array_values($categories_by_the_date);
}