Temps d'exécution

// VARIABLE GLOBALE
var base_url = "http://localhost/";


var Xhr = null;
var Result = "";


function getXhr(){ // Verification de l'explorateur
    if(window.XMLHttpRequest){
        Xhr = new XMLHttpRequest(); // IE7 + Mozilla + GoogleChrome...
    } else if(window.ActiveXObject) {
        Xhr = new ActiveXObject("MSXML2.HTTP.3.0"); // IE6 et inf�rieur
    }
}

function roundTrip(file,param){
    getXhr();
    Xhr.open("POST",file,true);
    Xhr.send("tri="+param);
    alert("fin GET");    // Alert permettant le retour (si abscente => retour vide)
    return Xhr.responseText;
}

function select_tri(data){
    alert(data);
    Result = roundTrip(base_url+'outils/xxx_xxx.php',data);
    alert(Result);
}

<?php
echo $_POST["tri"];
?>

	httpRequest.onreadystatechange = function()
	{
		if(httpRequest.readyState == 4 && httpRequest.status == 200)
		{
//alert(httpRequest.responseText);
}

}

function roundTrip(file,param){
    getXhr();
    Xhr.open("POST",file,true);
    Xhr.send("tri="+param);
    Xhr.onreadystatechange = function(){
        if(Xhr.readyState == 4){ //navigateur
            if(Xhr.status == 200){ //serveur
                return Xhr.responseText;
            	alert(Xhr.responseText);
            } else {
                switch(Xhr.status){
                    case 304 : "Champ 'if-modified-since' non modifie."; break;
                    case 403 : "Repertoire interdit."; break;
                    case 404 : "Page introuvable."; break;
                    case 500 : "Erreur de configuration"; break;
                    default : alert("Probleme Inconnu du Serveur"); break;
                }
            }
        }
    }
}