Абстрактный класс и наследование

<?php 
abstract class Lesson{
   private $duration;
   private $costStrategy;
   
   function __construct($duration, CostStrategy $strategy){
      $this->duration=$duration;
      $this->costStrategy=$strategy;
   }
   
   function cost(){
      return $this->costStrategy->cost($this);
   }
   
   function chatgeType(){
      return $this->costStrategy->chargeType();
   }
   
   function getDuration(){
      return $this->duration;
   }
}
 
class Lecture extends Lesson{
   
}
 
class Seminar extends Lesson{
}
 
abstract class CostStrategy{
   abstract function cost(Lesson $lesson);
   abstract function chargeType();
}
class TimedCostStrategy extends CostStrategy{
    function cost(Lesson $lesson){
       return($lesson->getDuration()*5);
    }
    
    function chargeType(){
       return "Po4asova9 oplata";
    }
}
 
class FixedCostStrategy extends CostStrategy{
   function cost(Lesson $lesson){
      return 30;
    }
    
   function chargeType(){
      return "Fiksirovanna9 oplata";
   }
}
 
$lessons[]= new Seminar(4, new TimedCostStrategy());
$lessons[]= new Lecture(4, new FixedCostStrategy());
 
foreach($lessons as $lesson){
   print "oplata za zan9tie {$lesson->cost()}.";
   print "Tip oplatbl:{$lesson->chargeType()}.";
}
?>