error public XML jquery cross domain with php

I need help!! well, the problem is that i can´t receive xml in javascript since php but i don´t know why doesn't work!

First the javascript since call php and send a variable to php

/* 
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function createObjetXML(url) {
    $.ajax({
        type: 'GET',
        url: url,
        dataType: "xml",
        success: function (xml) {
            xmlDoc = document.implementation.createDocument("", "", null);
            xmlDoc.load(xml);
        },
        error: function () {
            alert('Error');
        }
    });
}

function enviarVariableAxml(valor)
{
    var codigoPostal = valor;
    codigoPostal = codigoPostal.substring(3);
    codigoPostal=parseInt(codigoPostal);
 
    var url = "envioAemet.php?codigoPostal=" + codigoPostal;
   
    $.ajax.open("GET", url, true);
    $.ajax.send(null);
    createObjetXML(url);
}

function multiPeticion()
{
    var now = new Date();
    var hora = now.getHours();
    var arrayCodigo = ['cod39035', 'cod39075', 'cod39080'];
    $.each(arrayCodigo, function (i, valor) {
        enviarVariableAxml(valor);


    });
}

window.onload = function ()
{
    multiPeticion();
};

Second the php since call the public xml

<?php
    $codigoPostal=$_REQUEST['codigoPostal'];
    $url = $_REQUEST['http://ift.tt/1yJD5rr'+$codigoPostal+'.xml'];
    header("Content-type: text/xml");
    readfile($url);
?>

thanks!!