我需要javascript吗？在PHP？

Hey guys well I figured out how to style a php echo, which was quite common sense, but instead having an ugly colored box just waiting for an answer, is there a way to just display the box when the user hits the calculation they want? my website - http://codepen.io/willc86/pen/BrEmt though php wont work, just have it for styling purpose, but I am sure you will understand the concept.

this is my php/html

<html>
    <body>

        <style><?php include "style.css" ?></style>



        <form action="index.php" method="POST">

      <table border="1">

      <td>
        <p>insert value one: <input type="text" name="num1"> <br>
        <p>insert value two: <input type="text" name="num2"> <br>
      </td>

        <td>
            <input type="submit" name="add" value="Addition">
            <input type="submit" name="sub" value="Subtraction">
            <input type="submit" name="mult" value="Multiplication">
            <input type="submit" name="div" value="Division">
            <input type="submit" name="all" value="Display All">
        </td>
      </table>
        </form>



<div id="answer">

  hello
<?php

if (isset($_POST['num1']) && ($_POST['num2'])){
    $val1 = $_POST['num1'];
    $val2 = $_POST['num2']; 
    $add = $val1+$val2;
    $sub = $val1-$val2;
    $mult = $val1*$val2;
    $div = $val1/$val2; 
}

if (isset($_POST['add'])){
    echo $add;
}



?>  
        </div>      


    </body>
</html>

and this is my CSS

body{
  background-color:#f0f0f0;
}

table{
  margin:50px auto;
  background-color: tan;
  border: 2px solid black;
}

#answer{
margin:auto;  
    width:100px;
    height: 100px;
    background-color: tan;
text-align:center;
border: solid black 1px;
line-height: 100px;  
}

Put your tags within your PHP's if condition, like this:

<?php

if (isset($_POST['num1']) && ($_POST['num2'])){

    echo("<div id=\"answer\">");
    $val1 = $_POST['num1'];
    $val2 = $_POST['num2']; 
    $add = $val1+$val2;
    $sub = $val1-$val2;
    $mult = $val1*$val2;
    $div = $val1/$val2; 

if (isset($_POST['add'])){
    echo $add;
}

echo("</div>");

}

?>

我需要javascript吗？ 在PHP？

我需要javascript吗？在PHP？