function changeContent(pageName) {
  var xmlHttp;
  try {
    xmlHttp = new XMLHttpRequest();
  } catch(e) {
    try {
      xmlHttp = new ActiveXObject("Msxml12.XMLHTTP");
    } catch(e) {
      try {
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
      } catch(e) {
        alert("Sorry, your browser is not supported by this website");
        return false;
      }
    }
  }
  xmlHttp.onreadystatechange = function() {
    if (xmlHttp.readyState == 4) {
      document.getElementById('contentSection').innerHTML = xmlHttp.responseText;
    }
  }
  pageName += ".php";
  xmlHttp.open("GET",pageName,true);
  xmlHttp.send(null);
}