Ajax (prototype)

I'm using Prototype's AJAX to get the "alert" the response of a simple PHP script (a script that echoes a single line), but it doesn't seem to work. Here's the code:

function init()
{
var url = "http://localhost/script.php";
var myAjax = new Ajax.Request(
url,
{
onSuccess: showResponse
});
}

function showResponse(response)
{
alert(response.responseText);
}

window.onload = init;
The code has no errors, but still doesn't show the response even if i add an onFailure event handler. What could be wrong?