[PHP AJAX]Dynamically changing cart

[amii12]

Hello guys, i want to make dynamically changing cart in my shop sth similar like here (put any product in the cart): https://www.komputronik.pl/index.php/carts/pl/simpleCart.
I figure out sth like this, but when i click from LINE 1 nothing happens :

Simple php fragment which includes elements of cart

$html .= '<td><input type="TEXT" onFocus="this.select();" id="przycisk_quantity" name="quantity_'.$cartitem['id'].'" value="'.$cartitem['quantity'].'"></td>';

$html .= '<script type="text/javascript">
zmienna='.$cartitem['id'].';</script>';  //assign ID record to JS variable


$html .= '<img onClick="szukaj(zmienna);" src="layout/'.$style.'/cart_recount_off.png">'; //LINIE 1

JS script which should send data to the server

function szukaj(id) {
var wartosc = document.getElementById("przycisk_quantity").value;
var xhr = xmlhttp();
var plik = "baza.php";
xhr.open("GET", plik+"?parametr="+wartosc+"&id="+id, true);

xhr.onreadystatechange = function () {
 if(xhr.readyState == 4 && xhr.status == 200) {
 saveResult(xhr.responseText);
 }
 else {
 saveResult("There has been error: " + xhr.statusText); 
 }
};

xhr.send(null);
}



//function creating XMLHttp object in many other browsers
function xmlhttp() {
 if (typeof XMLHttpRequest != "undefined") {
 return new XMLHttpRequest();
 }
 else if (window.ActiveXobject) {
 var aVersions = ["MSXML2.XMLHttp.5.0", "MSXML2.XMLHttp.4.0", "MSXML2.XMLHttp.3.0", "MSXML2.XMLHttp", "MSXML2.XMLHttp", "Microsoft.XMLHttp"];

  for (var i=0; i<aVersions.length; i++)  {
   try {
   var oXmlHttp = new ActiveXObject(aVersions[i]);
   return oXmlHttp;
   }
   catch (oError) {}
  }
 }
throw new Error("Cant't create XMLHttp object.");
}


function saveResult(tresc) {
var box = document.getElementById("przycisk_quantity");
if (box.innerHTML != "") box.innerHTML = "";
box.innerHTML = tresc;
}

PHP database modyfing script

//connection with database

$parametr = $_GET['parametr'];   //gets the parameters from query string
$id = $_GET['id'];   

$zap = mysql_query("UPDATE cart SET quantity=".$parametr." WHERE id=".$id.";") or die('Error in UPDATE ' . mysql_error());
if (!$zap) echo 'There has been an error in UPDATE ' . mysql_error(); 
else {
$zap1 = mysql_query("SELECT id, quantity FROM cart WHERE id=".$id.";") or die('Error in SELECT ' . mysql_error());
if (!$zap1) echo 'There has been an error in SELECT ' . mysql_error();
else {
while ($row = mysql_fetch_array($zap1)) echo $row['quantity'];  //print out the updated results
}
}