public static function call()
 {
     if (self::$db_instance == null) {
         self::$db_instance = new mysqli(HOST, USERNAME, PWD, DBNAME);
         if (self::$db_instance->connect_errno) {
             echo "Failed to connect to MySQL: (" . self::$db_instance->connect_errno . ") " . self::$db_instance->db_instance->connect_error;
         }
     }
     return self::$db_instance;
 }