add more methods to class using include_once

Question

I have an script that was written by a different developer. I need to expand the class's capabilities. I want to write my own methods on a different .php file. and use something like include_once

I've tried adding this on the constructor like:

class confusing_developer {
    public function __construct(){
        $this->$id 'blah blah blah';
        include_once "myMods.php";

and my myMods.php continues like:

private function banana(){
....

However, this does not work. Do you guys know any other alternative??