function favoris(sujet_id,type,mode){
  var xhr=getXhr();

  xhr.onreadystatechange=function(){
    if(xhr.readyState==4 && xhr.status==200){
      var resultat=xhr.responseText;
      document.getElementById('favori_'+sujet_id).innerHTML=resultat;
    }
  }

  xhr.open("POST","xhr_favoris.php",true);
  xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');

  xhr.send("sujet_id="+sujet_id+"&type="+type+"&mode="+mode);
}