PHP singleton

singleton.php

class Manager
{
    private static $instance;


    public static function get_instance()
    {
        if (!isset(self::$instance)) {
            $class_name = __CLASS__;
            self::$instance = new $class_name;
        }

        return self::$instance;
    }

    public function hi()
    {
        echo 'hi';
    }

    public function __clone()
    {
        trigger_error('Clone is not allowed for singleton object.', E_USER_ERROR);
    }

}

Manager::get_instance()->hi();