var writedTexte = getXmlHttpRequestObject();

function makeRequest(url_ecrire,param) {
  writedTexte.open("POST", url_ecrire+param, true);
  writedTexte.send(null) ;
}

function ajout_achat() {
 var url_ecrire = 'php/ajout_achat.php';
 var postStr_reservation = "?id="+encodeURIComponent(document.getElementById("id_achat").value)+"&qte="+encodeURIComponent(document.getElementById("qte_ok").value)+"&type="+encodeURIComponent(document.getElementById("type_achat").value) ;
 makeRequest(url_ecrire, postStr_reservation);
}