I have 3 classes in WordPress (the question itself is unrelated to it):

class WP_Widget

class Theme_Widget extends WP_Widget

class Specific_Widget extends Theme_Widget

Essentially Theme_Widget contains some extension functions to the basic WP_Widget.

Inside Specific_Widget I call one of Theme_Widget's methods:

class Specific_Widget {

    function __construct() {
         $this->some_method_that_belongs_to_Theme_Widget();
    }
}

When I instantiate Specific_Widget, PHP throws a fatal error as follows:

Fatal error: Call to private method Theme_Widget::some_method_that_belongs_to_Theme_Widget() from context 'Specific_Widget' in ...

Do you have an idea as to how I can resolve this? This is the first time I've received this error from PHP. Could it be derive from WordPress itself?