Contributors do not work in JavaScript

Good morning Guys, I do not know if I'll be clear, but I have a problem here. I need to set the URL path in my JavaScript.

define('URL', 'http://' . $_SERVER['HTTP_HOST'] . '/sistema/');
$html      = str_replace('#URL#'    ,URL            ,$html);

I set this constant, and it works perfectly in HTML and PHP. But in Javascript it looks like it's a String, I've tried several ways, and I think it's easy to solve, but I'm having a lot of trouble. See the Javascript.

var url = '" . URL . "';

function criaAJAX() {
    if (window.XMLHttpRequest)
    {// Codigo para IE7+, Firefox, Chrome, Opera, Safari
        return new XMLHttpRequest();
    } else
    {// Codigo para IE6, IE5
        return new ActiveXObject("Microsoft.XMLHTTP");
    }
}

Thank you.