PHP OOP Classes and Objects

Program 1

<?php 
 class MyClass
 {
       public $m,$n,$r;  // instacne
      
        function add()
        {
            echo "<center>";
             $this->r=$this->m + $this->n;    
             echo "Addition is".$this->r;
             echo "</center>";
             return $this->r;
        }
        function sub()
        {
            echo "<center>";
             $this->r=$this->m - $this->n;    
            echo "Subtraction is".$this->r;
             echo "</center>";
             return $this->r;
        }
 }
$obj1=new MyClass();
 //$obj1->display();
  $obj1->m=600;
$obj1->n=500;
// $a=$obj1->add();
$obj1->add();
echo "<br>";
$obj1->sub();
// echo "<center>";
// echo "Addition is: ".$a;
// echo "</center>";