My problem is if I select more than one radio button sometimes both value gets stored. Why?

function ajax_value_save(id) {
  if (document.getElementById('a').checked) {
    var rate_value = document.getElementById('a').value;
  } else if (document.getElementById('b').checked) {
    var rate_value = document.getElementById('b').value;
  } else if (document.getElementById('c').checked) {
    var rate_value = document.getElementById('c').value;
  } else if (document.getElementById('d').checked) {
    var rate_value = document.getElementById('d').value;
  }
  var xmlhttp = new XMLHttpRequest();
  xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
      var result1 = document.getElementById('val1');
      result1.innerHTML = xmlhttp.responseText;
    }
  }
  xmlhttp.open('GET', 'answersave.php?valsave=' + rate_value + '&qid=' + id, true);
  xmlhttp.send();
}

if(!empty($_REQUEST['valsave'])){
$valsave = $_REQUEST['valsave'];
$id = $_REQUEST['qid'];
}
array_push($_SESSION['answers'], $valsave);
array_push($_SESSION['quid'], $id);