function refreshCaptcha(wdir) {

    var http_request = false;

    if (window.XMLHttpRequest) {
        http_request = new XMLHttpRequest();
    } else if (window.ActiveXObject) {
        try {
          http_request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (eror) {
          http_request = new ActiveXObject("Microsoft.XMLHTTP");
        }
    }

    http_request.onreadystatechange = function() { zpracujAjax(http_request); };
    http_request.open('GET', wdir+'/generateCaptcha.php', true);
    http_request.send(null);
}

function zpracujAjax(http_request) {
  if (http_request.readyState == 4) {
    if (http_request.status == 200) {
      document.getElementById("captchaImg").src = http_request.responseText;
    }
  }
}