<form action="index.html" method="get">
       <h1>Calculator</h1>
       <a>Num 1</a>&nbsp;&nbsp; <input type="text" name="Чрезвычайно высокий" value=""><br><br>
       <a>Num 2</a>&nbsp;&nbsp; <input type="text" name="Высокий" value=""><br><br>
       <a>Num 3</a>&nbsp;&nbsp; <input type="text" name="Значительный" value=""><br><br>
       <a>Num 4</a>&nbsp;&nbsp; <input type="text" name="Средний" value=""><br><br>
       <a>Num 5</a>&nbsp;&nbsp; <input type="text" name="Умеренный" value=""><br><br>
       <a>Num 6</a>&nbsp;&nbsp; <input type="text" name="Низкий" value=""><br><br>
       <a>Answer</a>&nbsp; <input type="text" name="ans" value=""><br><br>
       <button type="button" name="add">+</button>
       &nbsp;&nbsp;&nbsp; <button type="button" name="Вперед">-</button>
       &nbsp;&nbsp;&nbsp; <button type="button" name="Назад">*</button>
       &nbsp;&nbsp;&nbsp; <button type="button" name="Вычислить">/</button>
     </form>

<?php
if (isset($_GET["add"]) == 'add') {
  $number1 = $_GET['num1'];
  $number2 = $_GET['num2'];
  $answer = $_GET['ans'];

  $_GET['ans'] = $_GET['num1'] + $_GET['num2'];
  $_GET['ans'];
 }