Simple Category Enhancements 1.0

<?php
    /**
    * Plugin Name: Simple Category Enhancements by DM
    * Description: Minimalist WordPress plugin that adds simple enhancements to category page.
    * Version: 1.0
    * Author: Dawid Mikulski
    */

    /*
        Creating simple settings page for:
        - Subcategory list header
        - RSS text
    */

    // Add settings to the Customizer.
    function sce_customize_register($wp_customize) {
        // Section for plugin settings.
        $wp_customize->add_section('sce_settings_section', array(
            'title' => __('Simple Category Enhancements', 'sce'),
            'priority' => 30,
        ));

        // Setting for subcategory heading text.
        $wp_customize->add_setting('sce_subcategory_heading_text', array(
            'default' => __('Subcategories:', 'sce'),
            'sanitize_callback' => 'sanitize_text_field',
        ));

        $wp_customize->add_control('sce_subcategory_heading_text_control', array(
            'label' => __('Subcategory Heading Text', 'sce'),
            'section' => 'sce_settings_section',
            'settings' => 'sce_subcategory_heading_text',
            'type' => 'text',
        ));

        // Setting for RSS link text.
        $wp_customize->add_setting('sce_rss_link_text', array(
            'default' => __('Subscribe to RSS', 'sce'),
            'sanitize_callback' => 'sanitize_text_field',
        ));

        $wp_customize->add_control('sce_rss_link_text_control', array(
            'label' => __('RSS Link Text', 'sce'),
            'section' => 'sce_settings_section',
            'settings' => 'sce_rss_link_text',
            'type' => 'text',
        ));
    }

    /*
        Displaying list of subcategories under category
        description at category page.
    */
    
    function sce_display_subcategories_list($description) {
        if (is_category()) {
            // Get the current category.
            $current_category = get_queried_object();

            // Get subcategories.
            $subcategories = get_categories(array(
                'child_of' => $current_category->term_id,
                'hide_empty' => false,
            ));

            // Check if there are subcategories.
            if (!empty($subcategories)) {
                $heading_text = get_theme_mod('sce_subcategory_heading_text', __('Subcategories:', 'sce'));
                $output = '<div class="subcategory-list">';
                $output .= '<h3>' . esc_html($heading_text) . '</h3>';
                $output .= '<ul>';

                // Display each subcategory as a list item.
                foreach ($subcategories as $subcategory) {
                    $output .= '<li><a href="' . get_category_link($subcategory->term_id) . '">' . $subcategory->name . '</a></li>';
                }

                $output .= '</ul>';
                $output .= '</div>';

                // Append the subcategory list to the description.
                $description .= $output;
            }
        }
        return $description;
    }

    /*
        Creating 'a' element with RSS URL for current category
    */

    function sce_add_rss_icon($description) {
        if (is_category()) {
            $current_category = get_queried_object();
            $rss_url = get_category_feed_link($current_category->term_id);
            $rss_text = get_theme_mod('sce_rss_link_text', __('Subscribe to RSS', 'sce'));

            $icon = '<form action="'.esc_url($rss_url).'" method="get" target="_blank">
            <button type="submit"><span>
            <svg width="20" height="20" viewBox="0 0 20 20" aria-hidden="true">
            <path d="M17.9,0H2.1C1,0,0,1,0,2.1v15.7C0,19,1,20,2.1,20h15.7c1.2,0,2.1-1,2.1-2.1V2.1C20,1,19,0,17.9,0z M5,17.1c-1.2,0-2.1-1-2.1-2.1s1-2.1,2.1-2.1s2.1,1,2.1,2.1S6.2,17.1,5,17.1z M12,17.1h-1.5c-0.3,0-0.5-0.2-0.5-0.5c-0.2-3.6-3.1-6.4-6.7-6.7c-0.3,0-0.5-0.2-0.5-0.5V8c0-0.3,0.2-0.5,0.5-0.5c4.9,0.3,8.9,4.2,9.2,9.2C12.6,16.9,12.3,17.1,12,17.1L12,17.1z M16.6,17.1h-1.5c-0.3,0-0.5-0.2-0.5-0.5c-0.2-6.1-5.1-11-11.2-11.2c-0.3,0-0.5-0.2-0.5-0.5V3.4c0-0.3,0.2-0.5,0.5-0.5c7.5,0.3,13.5,6.3,13.8,13.8C17.2,16.9,16.9,17.1,16.6,17.1L16.6,17.1z"></path></svg></span> '.esc_attr($rss_text).'</button>
            </form>';

            $description .= $icon;
        }
        return $description;
    }

    /*
        Add functions
    */
    add_action('customize_register', 'sce_customize_register');
    add_filter('category_description', 'sce_add_rss_icon');
    add_filter('category_description', 'sce_display_subcategories_list');

?>