/**
  * Gets instance of class.
  *
  * @return WP_Google_DFP_Ads_Head Instance of the class.
  */
 public static function get_instance()
 {
     if (null == self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
<?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();