settings.php

<?php
require_once BSA_PATH . 'inc/constants.php';

class BsaSetting {

    // Create a setting submenu under option menu of WordPress admin site
    static function setting_menu() {
        add_options_page(__('BJH Website Assistant Settings', 'bjh-site-assistant'),
                         __('BJH Site Assistant', 'bjh-site-assistant'),
                         'administrator',
                         BsaConst::OPTIONS_PAGE,
                         array(__CLASS__, 'setting_page'));
    }

    // The main section of setting page
    static function setting_page() {
        // Reload the option settings
        global $bsa_options;
        $bsa_options = get_option(BsaConst::OPTIONS_GROUP);

        // Generate the setting page section
        echo '
        <div class="wrap">
            <h2>' . __('BJH Website Assistant Settings', 'bjh-site-assistant') . '</h2>
            <div class="narrow">
                <form action="options.php" method="post">
                    <p>' . __('Thanks for using BJH Website Assistant. This plugin provides small functions'
                            . ' to help your website working better.', 'bjh-site-assistant') . '</p>';
                    // The option to be submitted by form
                    settings_fields(BsaConst::OPTIONS_GROUP);
                    // List the registered settings to page
                    do_settings_sections(BsaConst::OPTIONS_PAGE);
                    echo '<p class="submit">
                        <input name="submit" type="submit" class="button-primary" value="'
                         . __('Save Change', 'bjh-site-assistant') . '" />
                    </p>
                </form>
            </div>
        </div>';
    }

    // Initialize the settings by register setting option and fields
    static function setting_init() {
        register_setting(BsaConst::OPTIONS_GROUP, BsaConst::OPTIONS_GROUP);
        add_settings_section(BsaConst::OPTIONS_SECTION,
                             __('Basic Settings', 'bjh-site-assistant'),
                             array(__CLASS__, 'setting_section'),
                             BsaConst::OPTIONS_PAGE);
    }

    // The message to be displayed on top of setting section
    static function setting_section() {
        echo '<p>' . __('Please enable or disable the functions by clicking the check box below.'
                      , 'bjh-site-assistant') . '</p>';
    }

    // Register and include admin JS script for the plugin
    static function admin_script() {
        $utilJs = BSA_URL . '/js/util.js';
        // Register the JS, the first arg "util.js" will be used in wp_enqueue_script()
        wp_register_script('util.js', $utilJs);
        // Safely add JS to wordpress pages
        wp_enqueue_script( 'util.js');
    }

    // Register and include admin CSS style for the plugin
    static function admin_styles() {
        $styleCss = BSA_URL . '/css/style.css';
        wp_register_style('style.css', $styleCss);
        wp_enqueue_style('style.css');
    }
}

// Include JS scriopt to wordpress admin backend
add_action('admin_print_scripts', 'BsaSetting::admin_script');
// Include CSS style to wordpress admin backend
add_action('admin_print_styles', 'BsaSetting::admin_styles');

// Init the admin option field
add_action('admin_init', 'BsaSetting::setting_init');

// Add the admin option menu section
add_action('admin_menu', 'BsaSetting::setting_menu');