Ajax Request

<?php
function calcolo(array $data)
{
    $math = array();

    // ... make math.

    return $math;
}

function retrieveData()
{
    // Prevent tainted data.
    $data = array();

    // Data doesn't exists? Just return empty array.
    // Nothing else to do.
    if(!isset($_POST['data']) {
        return array();
    }

    // Parse Data.
    parse_str($_POST['data'], $data);

    // See http://php.net/manual/it/function.filter-var.php
    // See http://php.net/manual/it/function.filter-var-array.php
    // See http://php.net/manual/it/filter.filters.sanitize.php
    $data = filter_var_array($data[$key], array(
        // Sanitize Costo, but I'll use https://github.com/moneyphp/money
        // We are talking about money afterall.
        'costo'   => FILTER_SANITIZE_NUMBER_FLOAT,
        'ore'     => FILTER_SANITIZE_NUMBER_INT,
        'giorni'  => FILTER_SANITIZE_NUMBER_INT,
        'potenza' => FILTER_SANITIZE_STRING,
        'luce'    => FILTER_SANITIZE_STRING,
    ));

    return $data;
}

function makeHTML(array $data)
{
    $html = '';

    // ... create the HTML output to send to `htmlSend()`

    return $html;
}

function htmlSend($html)
{
    $data = array('html' => $html);

    // If empty html
    if('' === $html) {
        // @see https://codex.wordpress.org/Function_Reference/wp_send_json_error
        wp_send_json_error($data);
    }

    // @see https://codex.wordpress.org/Function_Reference/wp_send_json_success
    wp_send_json_success($data);
}

function checkAjaxRequest()
{
    // Verify request.
    // See https://codex.wordpress.org/Function_Reference/check_ajax_referer
    // Will die if not verified.
    check_ajax_referer('my-special-string', 'security', true);
}

/**
 * Controller
 *
 * This manage the request and send the output via json.
 */
function controller()
{
    // Do nothing if not an ajax request.
    if(!wp_doing_ajax()) {
        return;
    }

    // Will die if the nonce has not been verified.
    checkAjaxRequest();

    $output = '';
    $data   = retrieveData();

    if($data) {
        $output = makeHTML(calcolo($data));
    }

    // Then send the output to `htmlSend` that will send it wrapped into a json.
    htmlSend($output);
}