function Participation(id_concours){
  var xhr=getXhr();

  xhr.onreadystatechange=function(){
    if(xhr.readyState==4 && xhr.status==200){
      var docXML=xhr.responseXML;
      var reponse=docXML.getElementsByTagName('reponse');
      var etat=reponse.item(0).getElementsByTagName('etat').item(0).firstChild.data;
      var nb_participations=reponse.item(0).getElementsByTagName('nb_participations').item(0).firstChild.data;
      var nb_participations_s=reponse.item(0).getElementsByTagName('nb_participations_s').item(0).firstChild.data;

      if (etat=='O'){
        document.getElementById('case_'+id_concours).src='images/i-case0.png';
      }
      else if (etat=='X'){
        document.getElementById('case_'+id_concours).src='images/i-case1.png';
      }

      document.getElementById('nb_participations').innerHTML=nb_participations;
      document.getElementById('nb_participations_s').innerHTML=nb_participations_s;
    }
  }

  xhr.open("POST","participation.php",true);
  xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
  var participations=document.getElementById('nb_participations').innerHTML;
  xhr.send("id_concours="+id_concours+"&participations="+participations);
}