OOP echo $this->title = $part VS echo $par

大家好，我开始学习PHP OOP。我有个问题。在下面的例子中，为什么我需要声明var$price，var$title；并将其用于echo$this->title=$par；。其中我可以直接回显$par

        <?php
        class  Books{
            /* Member variables */
            var $price;
            var $title;
            /* Member functions */
            function setPrice($par){
               $this->price = $par;
            }
            function getPrice(){
               echo $this->price ."<br/>";
            }
            function setTitle($par){
              echo $this->title = $par;
              //echo $par;  what is the difference between the two.
            }
            function getTitle(){
               echo $this->title ." <br/>";
            }
        }
        ?>
        <?php
      $physics = new Books;
       $maths = new Books;
       $chemistry = new Books;
         $physics->setTitle( "Physics for High School" );
       $chemistry->setTitle( "Advanced Chemistry" );
       $maths->setTitle( "Algebra" );
   $physics->setPrice( 10 );
   $chemistry->setPrice( 15 );
   $maths->setPrice( 7 );

    $physics->getTitle();
   $chemistry->getTitle();
   $maths->getTitle();
   $physics->getPrice();
   $chemistry->getPrice();
   $maths->getPrice();