Singleton pattern causes error: Using $ this when not in object context

I made this Singleton pattern that I saw on a site and found it great, well what I wanted. However something is going wrong because I have the error:

Fatal error: Uncaught Error: Using $this when not in object context in /opt/lampp/htdocs/myschool/DAO/Conncetion.php:37
Stack trace: #0 /opt/lampp/htdocs/myschool/Login.php(18):DAO\Connection::getInstance() #1 /opt/lampp/htdocs/myschool/login.php(5):Login->loginconnect() #2 {main} thrown in /opt/lampp/htdocs/myschool
/DAO/Conncetion.php on line 37

And if I put function __construct () {} as private or protected then yes for that reason. I've read about using $ this in a context where it is not an object. However I want to use this command because of the idea of loading the variables dynamically by the functions.

This is the class:

class Connection
{
    public static $instance;
    private static $dbtype   = "mysql";
    private static $host     = "localhost";
    private static $port     = "3306";
    private static $user     = "root";
    private static $password = "";
    private static $db       = "school";

    /*Metodos que trazem o conteudo da variavel desejada
     @return   $xxx = conteudo da variavel solicitada*/
    private static function getDBType()  {return self::$dbtype;}
    private static function getHost()    {return self::$host;}
    private static function getPort()    {return self::$port;}
    private static function getUser()    {return self::$user;}
    private static function getPassword(){return self::$password;}
    private static function getDB()      {return self::$db;}

     function __construct() {

    }

    public static function getInstance()
    {

     if(!isset(self::$instance)){
         self::$instance = new PDO($this->getDBType().":host=".$this->getHost().";port=".$this->getPort().";dbname=".$this->getDB(), $this->getUser(), $this->getPassword());
            self::$instance->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            self::$instance->setAttribute(PDO::ATTR_ORACLE_NULLS, PDO::NULL_EMPTY_STRING);

        }

        return self::$instance;
    }

    private function __clone()
    {
    }
    private function __wakeup()
    {
    }
}