/**
  * Main Mailtpl Instance
  *
  * Ensures only one instance of Mailtpl is loaded or can be loaded.
  *
  * @since 1.0.0
  * @static
  * @see Mailtpl()
  * @return Mailtpl - Main instance
  */
 public static function instance()
 {
     if (is_null(self::$_instance)) {
         self::$_instance = new self();
     }
     return self::$_instance;
 }
/**
 * Begins execution of the plugin.
 * @since    1.0.0
 */
function Mailtpl()
{
    $plugin = Mailtpl::instance();
    $plugin->run();
}
<?php

$settings = Mailtpl::opts();
include_once 'partials/header.php';
include_once 'partials/email-content.php';
include_once 'partials/footer.php';
 /**
  * Initialize the class and set its properties.
  *
  * @since    1.0.0
  * @param      string    $plugin_name       The name of this plugin.
  * @param      string    $version    The version of this plugin.
  */
 public function __construct($plugin_name, $version)
 {
     $this->plugin_name = $plugin_name;
     $this->version = $version;
     $this->defaults = Mailtpl::defaults();
 }