I have an HTML document, which loads content from a PHP file using an AJAX call. The important bit of my code is below:

**default.html :**

```
/*more code above*/
var PHP_URL = "content.php";
var Content = document.getElementById('Content');
ajaxRequest = new XMLHttpRequest();
ajaxRequest.onreadystatechange =
    function() {
        if(ajaxRequest.readyState==4) {
            if (ajaxRequest.status==200)
                Content.innerHTML = ajaxRequest.responseText;
            else
                Content.innerHTML = "Error:<br/>unable to load page at <b>"+PHP_URL+"</b>";
            Content.className = "Content Solid";
        }
    }
ajaxRequest.open("GET",PHP_URL,true);
ajaxRequest.send();
/*more code below*/

```

Is it possible for the file at 'content.php' to detect that it has been called from 'default.html', or a different calling document as necessary?