Airplane_Mode_Core::getInstance PHP Method

        public static function getInstance()
        {
            if (!self::$instance) {
                self::$instance = new self();
            }
            return self::$instance;
        }

/**
 * Check if AJAX works as expected
 *
 * @since 2.2
 * @return bool True if AJAX works, false otherwise
 */
function edd_test_ajax_works()
{
    // Check if the Airplane Mode plugin is installed
    if (class_exists('Airplane_Mode_Core')) {
        $airplane = Airplane_Mode_Core::getInstance();
        if (method_exists($airplane, 'enabled')) {
            if ($airplane->enabled()) {
                return true;
            }
        } else {
            if ($airplane->check_status() == 'on') {
                return true;
            }
        }
    }
    add_filter('block_local_requests', '__return_false');
    if (get_transient('_edd_ajax_works')) {
        return true;
    }
    $params = array('sslverify' => false, 'timeout' => 30, 'body' => array('action' => 'edd_test_ajax'));
    $ajax = wp_remote_post(edd_get_ajax_url(), $params);
    $works = true;
    if (is_wp_error($ajax)) {
        $works = false;
    } else {
        if (empty($ajax['response'])) {
            $works = false;
        }
        if (empty($ajax['response']['code']) || 200 !== (int) $ajax['response']['code']) {
            $works = false;
        }
        if (empty($ajax['response']['message']) || 'OK' !== $ajax['response']['message']) {
            $works = false;
        }
        if (!isset($ajax['body']) || 0 !== (int) $ajax['body']) {
            $works = false;
        }
    }
    if ($works) {
        set_transient('_edd_ajax_works', '1', DAY_IN_SECONDS);
    }
    return $works;
}