Programmatically Creating Custom Post Types, Taxonomies, and Fields in WordPress: A Step-by-Step Guide for Dynamic Content Structures.

Below is an example of how you can create a custom post type with a custom taxonomy and custom fields programmatically in WordPress. This code can be added to your theme's functions.php file or a custom plugin.

<?php
// Step 1: Register Custom Post Type

function custom_post_type() {
    $labels = array(
        'name'               => _x('Books', 'post type general name'),
        'singular_name'      => _x('Book', 'post type singular name'),
        'add_new'            => _x('Add New', 'book'),
        'add_new_item'       => __('Add New Book'),
        'edit_item'          => __('Edit Book'),
        'new_item'           => __('New Book'),
        'all_items'          => __('All Books'),
        'view_item'          => __('View Book'),
        'search_items'       => __('Search Books'),
        'not_found'          => __('No books found'),
        'not_found_in_trash' => __('No books found in the Trash'),
        'parent_item_colon'  => '',
        'menu_name'          => 'Books'
    );

    $args = array(
        'labels'        => $labels,
        'public'        => true,
        'has_archive'   => true,
        'rewrite'       => array('slug' => 'books'),
        'menu_icon'     => 'dashicons-book', // You can choose an icon from Dashicons: https://developer.wordpress.org/resource/dashicons/
        'supports'      => array('title', 'editor', 'thumbnail', 'custom-fields'),
    );

    register_post_type('book', $args);
}

add_action('init', 'custom_post_type');

// Step 2: Register Custom Taxonomy

function custom_taxonomy() {
    $labels = array(
        'name'              => _x('Genres', 'taxonomy general name'),
        'singular_name'     => _x('Genre', 'taxonomy singular name'),
        'search_items'      => __('Search Genres'),
        'all_items'         => __('All Genres'),
        'parent_item'       => __('Parent Genre'),
        'parent_item_colon' => __('Parent Genre:'),
        'edit_item'         => __('Edit Genre'),
        'update_item'       => __('Update Genre'),
        'add_new_item'      => __('Add New Genre'),
        'new_item_name'     => __('New Genre Name'),
        'menu_name'         => __('Genre'),
    );

    $args = array(
        'hierarchical'      => true,
        'labels'            => $labels,
        'show_ui'           => true,
        'show_admin_column' => true,
        'query_var'         => true,
        'rewrite'           => array('slug' => 'genre'),
    );

    register_taxonomy('genre', array('book'), $args);
}

add_action('init', 'custom_taxonomy');

// Step 3: Add Custom Fields

function custom_fields() {
    add_meta_box('book_meta_box', 'Book Details', 'display_book_meta_box', 'book', 'normal', 'high');
}

function display_book_meta_box($post) {
    // Retrieve current values of custom fields
    $author = get_post_meta($post->ID, '_author', true);
    $published_year = get_post_meta($post->ID, '_published_year', true);

    // Output fields
    ?>
    <p>
        <label for="author">Author:</label>
        <input type="text" id="author" name="author" value="<?php echo esc_attr($author); ?>">
    </p>
    <p>
        <label for="published_year">Published Year:</label>
        <input type="text" id="published_year" name="published_year" value="<?php echo esc_attr($published_year); ?>">
    </p>
    <?php
}

function save_book_meta_box($post_id) {
    // Save custom field values
    if (isset($_POST['author'])) {
        update_post_meta($post_id, '_author', sanitize_text_field($_POST['author']));
    }

    if (isset($_POST['published_year'])) {
        update_post_meta($post_id, '_published_year', sanitize_text_field($_POST['published_year']));
    }
}

add_action('add_meta_boxes', 'custom_fields');
add_action('save_post', 'save_book_meta_box');

?>

Explanation:

1. Custom Post Type Registration (custom_post_type function):
   • Registers a custom post type named "Book" with custom labels and options.
   • Supports various features such as title, editor, thumbnail, and custom fields.
2. Custom Taxonomy Registration (custom_taxonomy function):
   • Registers a custom taxonomy named "Genre" associated with the "Book" post type.
   • Provides hierarchical taxonomy structure with custom labels.
3. Custom Fields (custom_fields function):
   • Adds a meta box for custom fields (Author and Published Year) on the "Book" post type editor.
   • Defines fields and their HTML markup.
4. Display and Save Custom Fields (display_book_meta_box and save_book_meta_box functions):
   • Displays the custom fields on the post editor.
   • Saves the custom field values when the post is saved.

This code creates a custom post type "Book" with a custom taxonomy "Genre" and adds custom fields (Author and Published Year). Adjust the field names, post type, and taxonomy details according to your specific requirements.