function getXhr(){
  var xhr=null; 
  if (window.XMLHttpRequest) xhr=new XMLHttpRequest(); 
  else if (window.ActiveXObject){
    try{
      xhr=new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e){
      xhr=new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
  else{
    alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest..."); 
    xhr=false; 
  } 
  return xhr;
}