OOP - Calling methods from classes that are inheriting?

Hi, How do I call methods from classes that are inherited? I want to add a mysql_num_rows() function to a second class that is an addon to a MySQL query in the first class. The first class will be used in several different classes for different parts of the site so I don't want to directly add the num_rows to the first class method as the query won't just be used for this function. Hope I have made myself as clear as possible! Cheers!