Libraries Extending Question

The "MY_" convention you use is the intended convention for extending CodeIgniter's core libraries. You are misusing it. See https://www.codeigniter.com/user_guide/g...core-class

The approach might be reasonable, with slightly different naming. You could make your class "Custom_parsedown", and start it with

Code:

require APPPATH . 'libraries/Parsedown.php';

class Custom_parsedown extends Parsedown {