function menu(link) {
  var xmlhttp;
  if (window.XMLHttpRequest) {
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
  } else if (window.ActiveXObject) {
    // code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  } else {
    alert("Your browser does not support XMLHTTP!");
  }

  xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4) {
      var content = document.getElementById("content");
      content.innerHTML = xmlhttp.responseText;
    }
  }

  xmlhttp.open("GET", link + ".php", true);
  xmlhttp.send(null);
}