JQuery Ajax request return error and status 0

I am using this pseudo class to make an Ajax request to the server:

function RequestManager(url, params, success, error){
    //Save this Ajax configuration
    this._ajaxCall = null;
    this._url= url;
    this._type = params.type;
    this._success = function(){
        alert("ok");
    };
    this._error = function(){
        alert("ko");
    };
}

RequestManager.prototype = {
    require : function(text){
        var self = this;
        this._ajaxCall = $.ajax({
            url: this._url,
            type: this._type,
            data: text,
            success: function(xmlResponse){
                var responseArray = [];
                var response = _extractElements(xmlResponse, arrayResponse);
                self._success(response);
            },
            error: self._error,
            complete : function(xhr, statusText){
                alert(xhr.status);
                return null;
            }
        });
    }

      

        
        
        
      

    

This is the PHP that will be loaded:

<?php
    header('Content-type: text/xml');

    //Do something
    $done = true;

    $response = buildXML($done);
    $xmlString = $response->saveXML();
    echo $xmlString;

    function buildXML ($done){
        $response = new SimpleXMLElement("<response></response>");
        if($done){
            $response->addChild('outcome','ok');
        }
        else {
            $response->addChild('outcome', 'ko');
        }
        return $response;
    }

      

        
        
        
      

    

When I instantiate a new object and use it to load the request, it always returns me an error and the status code is 0. The server is generating the XML document correctly. Why can't I get the correct 200 code?