在非对象上调用成员函数xpath()？

我正在尝试使用simpleXML从网站上获取图像,并且我收到一条PHP错误,说我正在尝试调用xpath()非对象上的成员函数.

下面是我试图用来获取图像源标记的行:

$xpath = '/html/body/div/div/div[5]/div/div/div[2]/div/div[2]/img';          
$html = new DOMDocument();
@$html->loadHTMLFile($target_URL);
$xml = simplexml_import_dom($html);   
$source_image = $xml->xpath($xpath);
$source_image = $source_image[0]['src'];

我究竟做错了什么？很明显倒数第二行有问题,但我不确定它是什么.

尝试此代码首先确保正确解析文档.

$xpath = '/html/body/div/div/div[5]/div/div/div[2]/div/div[2]/img';          
$html = new DOMDocument();
@$html->loadHTMLFile($target_URL);
$xml = simplexml_import_dom($html);   
if (!$xml) {
    echo 'Error while parsing the document';
    exit;
}
$source_image = $xml->xpath($xpath);
$source_image = $source_image[0]['src'];