public static function connect()
 {
     if (self::$conn == null) {
         try {
             SELF::$conn = new PDO("mysql:host=" . SELF::$server . ";dbname=" . SELF::$database, SELF::$username, SELF::$password);
         } catch (PDOException $e) {
             die($e->getMessage());
         }
     }
     return self::$conn;
 }