<form method="post">
    <input type="text" name="a" size=5> +
    <input type="text" name="n" size=5>
    <input type="submit" name="total">
</form>
<?php
function total()
{
    if (isset($_POST['total'])) {
        $a = $_POST["a"];
        $n = $_POST["n"];
        $vys = $a + $n;
        echo "<br>$a + $n = $vys";
    }
}
total();