function A() {
    this.b = 13;
    function finish(context,response) {
        context.b = response;
    }
    ajax(finish,this);
}

A.prototype = {
    constructor: A
}

function ajax(callback,context) {
    var http = new XMLHttpRequest();
    var url = "phppage.php";
    http.open("GET", url, true);
    http.onreadystatechange = recall;
    function recall() {
        alert('a');
        if(http.readyState == 4 && http.status == 200) {
            callback(context,http.responseText);
        }
    }
    http.send(null);
}

var d = new A();
alert(d.b);

function A() {
    this.b = 13;
    //this is how you should be storing contexts
    var that = this;
    function finish(response) {
        that.b = response;
    }
    ajax(finish);
}

function ajax(callback) {
    var http = new XMLHttpRequest();
    var url = "phppage.php";
    http.open("GET", url, true);
    http.onreadystatechange = function() {
        alert('a');
        if(http.readyState == 4 && http.status == 200) {
            callback(http.responseText);
        }
    }
    http.send(null);
}

http.open("GET", url, false);