我写了以下代码：
$xmldom = new DOMDocument();
$xmldom->load('http://www.robot-domestici.it/joomla/component/virtuemart/robot-domestici/robot-pulizia-casa?page=shop.feed');
但加载的文档为空（此类别不包含任何产品），并发生以下错误：
Warning: DOMDocument::load(): Document is empty in http://www.robot-domestici.it/joomla/component/virtuemart/robot-domestici/robot-pulizia-casa?page=shop.feed, line: 1 in C:\Users\Jacopo\Dropbox\Tirocinio\xampp-portable\htdocs\sites\prova\cerca categorie2.php on line 73
Warning: DOMDocument::load(): Start tag expected, '<' not found in http://www.robot-domestici.it/joomla/component/virtuemart/robot-domestici/robot-pulizia-casa?page=shop.feed, line: 1 in C:\Users\Jacopo\Dropbox\Tirocinio\xampp-portable\htdocs\sites\prova\cerca categorie2.php on line 73
并中断执行。很明显，URL是以dinamically方式生成的，甚至Feed的内容也可以改变，所以我不知道哪个URL是空的。我要做一些检查，但我无法弄清楚我能做些什么.... 我试图将Feed存储在变量中，但它不起作用：
$str=file_get_contents('http://www.robot-domestici.it/joomla/component/virtuemart/robot-domestici/robot-pulizia-casa?page=shop.feed');
if($str==='')
return;
我能做什么？