var XMLHttpRequestObject = false;
                   if (window.XMLHttpRequest){
                       XMLHttpRequestObject = new XMLHttpRequest();
                   } else if (window.ActiveXObject){
                       XMLHttpRequestObject = new ActiveXObject("Microsoft.XMLHTTP");
                   }
   function getData(dataSource, divID, cena)
                   {
                       if(XMLHttpRequestObject) {
                           var obj = document.getElementById(divID);
                           XMLHttpRequestObject.open("POST", dataSource);
                           XMLHttpRequestObject.setRequestHeader('Content-type','application/x-www-form-urlencoded');
                          
                           if(XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200)
                           {
                               obj.innerHTML = XMLHttpRequestObject.responseText;
                           }
                           XMLHttpRequestObject.send("cena=" + cena);
                       }
                   }

<form>
                   <div id="badania"><div class="otwBadania">Badania</div></div>
                   <div><input type="button" value="Przelicz" onclick="getData('cena.php', 'sCena', 1)"></div>
               </form>
               <div class="did">
                   <div id="suma">Kwota do zapłaty:<div id="sCena"></div></div>
               </div>

<?php
       if($_POST["cena"] == "1"){
           echo 'ddd';
       }
   ?>

if(XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200)
{
      obj.innerHTML = XMLHttpRequestObject.responseText;
}

XMLHttpRequestObject.onreadystatechange = function()
{
     if(XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200)
     {
          obj.innerHTML = XMLHttpRequestObject.responseText;
     }
}