<?php

function task95(...$args) {
    if (!in_array(count($args), [1, 2]))
        throw new Exception('Incorrect parameters count');
    $radiusesProduct = count($args) === 1 ? pow($args[0], 2) / 16 : $args[0] * $args[1];
    return 2 * pi() * $radiusesProduct;
}

function test(...$args) {
    printf("%s => %s\n", implode(', ', $args), call_user_func_array('task95', $args));
}

foreach([[15, 20], [15, 15], [60]] as $data) test(...$data);

/* результаты тестирования
15, 20 => 1884.9555921539
15, 15 => 1413.7166941154
60 => 1413.7166941154
*/