calculator (php)

<html>
  <body>

  <?php
  if(isset( $_GET["num1"])||isset( $_GET["num2"]))
  {
     if( $_GET["num1"] || $_GET["num2"] ) {
        echo "number 1 is ". $_GET['num1']. "<br /><br />";
        echo "number 2 is ". $_GET['num2']. "<br/>";
		$sum = $_GET["num1"] + $_GET["num2"];
		$pro = $_GET["num1"] * $_GET["num2"];
		$diff = $_GET["num1"] - $_GET["num2"];
		echo "the sum of two numbers is". $sum."<br/><br/>";
		echo "the  prod of two numbers is". $pro."<br/><br/>";
		echo "the diff of two numbers is". $diff;

        exit();
     }

  }

  ?>
  <html>
     <body>

        <form action = "<?php $_PHP_SELF ?>" method = "GET">
           enter first number : <input type = "text"  name = "num1" /><br/><br/>
           enter second number : <input type = "text" name = "num2" /><br/>
           <input type = "submit" name ="submit" value ="click me"/>
        </form>

     </body>
  </html>