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*/

“content.PHP”上的文件是否可以检测到它已从“default.html”调用,还是根据需要调用不同的调用文件？