var http_request = false;
function checkQuantity(pid, aid) {
  if (window.XMLHttpRequest) {
    http_request = new XMLHttpRequest();
    if (http_request.overrideMimeType) {
      http_request.overrideMimeType('text/xml');
    }
  } else if (window.ActiveXObject) {
    try {
      http_request = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
      try {
        http_request = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) {}
    }
  }
  if (!http_request) {
    alert('Nie mogę zainstalować obiektu XMLHTTP');
    return false;
  }
  http_request.onreadystatechange = function() {  printQuantity(http_request); };
  http_request.open('GET', 'checkQuantity.php?pid='+pid+'&aid='+aid, true);
  http_request.send(null);
}

function printQuantity(http_request) {
  if (http_request.readyState == 4) {
    if (http_request.status == 200) {
      var xmldoc = http_request.responseXML;
      var root_node = xmldoc.getElementsByTagName('content').item(0);
      var html_node = root_node.getElementsByTagName('html').item(0);
      var stan = html_node.getElementsByTagName('item').item(0).firstChild.data;
      document.getElementById("availability").innerHTML = stan;
    } else {
      alert("Błąd");
    }
  }
}