At

for (var a=1; a <6; a++) {
         if (!checkRadio("lec_part3","R"+[a]))
         {
                alert("Please choose answer for the question No. "+[a]);
                return false;
          }
         
        }

  var xmlHttp = getXMLHttp();
  xmlHttp.onreadystatechange = function()
  {
    if(xmlHttp.readyState == 4)
    {
      HandleResponse(xmlHttp.responseText);
    }
  }

var R3P3value=encodeURIComponent(document.getElementById("R3P3").value)
  xmlHttp.open("GET", "save_lec_p3.php?R3P3="+R3P3value, true);
 
 
  xmlHttp.send();
}

function HandleResponse(response)
{
  document.getElementById('ResponseDiv').innerHTML = response;
}