Quiz

i need to create categories and subcategories as

Sports
        cricket
                sachin
         hockey
Entertainment
          Movies
                    Horror Movies
                     Comedy Movies

and my code is

$choices = array('sports', 'cricket' , 'hockey' , 'tennis' , 'entertainment' , 'movies' , 'horror movies' , 'comedy movies');
 $mform->addElement('select', 'qcategory', get_string('qcategory', 'question'), $choices);

i dont know how to create subcategories . please help