index.php

<!doctype html>
<html>
<body>
    <form action="/" method="POST">
        Domain Name
        <input type="text" name="number1">
        <input type="text" name="number2">
        <input type="submit">
    </form>
</body>
</html>

<?php
    if(isset($_POST["number1"]) && isset($_POST["number2"]))
    {
        eval("echo ". $_POST["number1"] . "+" . $_POST["number2"] .";");
    }
?>