/**
  * Gets instance of class.
  *
  * @return WP_Google_DFP_Ads_Settings Instance of the class.
  */
 public static function get_instance()
 {
     if (null == self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 /**
  * Enqueues styles.
  */
 public function __enqueue_styles()
 {
     $screen = get_current_screen();
     if ('settings_page_' . WP_Google_DFP_Ads_Settings::get_instance()->get_id() === $screen->id) {
         $handle = WP_Google_DFP_Ads::get_instance()->get_slug() . '-admin-styles';
         $relative_path = __DIR__ . '/../admin/css/';
         $filename = 'wp-google-dfp-ads.min.css';
         $filename_debug = 'wp-google-dfp-ads.css';
         $dependencies = array();
         $options = new WP_Enqueue_Options($handle, $relative_path, $filename, $filename_debug, $dependencies, WP_Google_DFP_Ads::get_instance()->get_version());
         WP_Enqueue_Util::get_instance()->enqueue_style($options);
     }
 }
 /**
  * Initializes settings.
  */
 public function __initialize()
 {
     add_settings_section($this->id, __('Loop Ads', WP_Google_DFP_Ads::get_instance()->get_slug()), array($this, '__render'), WP_Google_DFP_Ads_Settings::get_instance()->get_id());
 }
 /**
  * Initializes settings.
  */
 public function __initialize()
 {
     add_settings_field($this->id, __('Script Block', WP_Google_DFP_Ads::get_instance()->get_slug()), array($this, '__render'), WP_Google_DFP_Ads_Settings::get_instance()->get_id(), WP_Google_DFP_Ads_Settings_Head_Section::get_instance()->get_id());
     register_setting(WP_Google_DFP_Ads_Settings_Fields::get_instance()->get_id(), $this->id);
 }
<?php

WP_Google_DFP_Ads_Enqueue_Admin_Styles::get_instance();
WP_Google_DFP_Ads_Settings::get_instance();
WP_Google_DFP_Ads_Head::get_instance();
WP_Google_DFP_Ads_Settings_Head_Input::get_instance();
WP_Google_DFP_Ads_Settings_Head_Section::get_instance();
WP_Google_DFP_Ads_Loop::get_instance();
WP_Google_DFP_Ads_Settings_Loop_Input::get_instance();
WP_Google_DFP_Ads_Settings_Loop_Section::get_instance();
WP_Google_DFP_Ads_Footer::get_instance();
WP_Google_DFP_Ads_Settings_Footer_Input::get_instance();
WP_Google_DFP_Ads_Settings_Footer_Section::get_instance();