Quelque chose

function populate_dest(f) {
  var l1    = f.elements["id_type_rdv"];
  var l2    = f.elements["id_dest"];
  var index = l1.selectedIndex;
  if(index < 1)
    l2.options.length = 0;
  else {
    var xhr_object = null;

    if(window.XMLHttpRequest) // Firefox
      xhr_object = new XMLHttpRequest();
    else if(window.ActiveXObject) // Internet Explorer
      xhr_object = new ActiveXObject("Microsoft.XMLHTTP");
    else { // XMLHttpRequest non supporté par le navigateur
      alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest...");
      return;
    }

    switch(l1.value) {
      case 1: var page = "phpToJs/tasks.inc.php"; break;
      case 2: var page = "phpToJs/agences.inc.php"; break;
      case 3: var page = "phpToJs/particuliers.inc.php"; break;
      case 4: var page = "phpToJs/prospects.inc.php"; break;
      case 5: var page = "phpToJs/users.inc.php"; break;
      case 6: var page = "phpToJs/fours.inc.php"; break;
      case 7: var page = "phpToJs/societes.inc.php"; break;
    }

    xhr_object.open("POST", page, true);

    xhr_object.onreadystatechange = function() {
      if(xhr_object.readyState == 4)
        eval(xhr_object.responseText);
    }

    xhr_object.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    var data = "id_type_rdv="+escape(l1.options[index].value)+"&form="+f.name+"&select=id_dest";
    xhr_object.send(data);
  }
}