PHP - DOMXpath - Get the value

我的代码:

    $doc = new DOMDocument;
    $doc->loadHTML($source);
    $xpath = new DOMXPath($doc);
    $result = $xpath->evaluate($xpath);
foreach($result as $node) {
     echo $node->nodeValue;
    }

//I'm trying to get the href attribute in: <a href="http://example.com/login">Log In</a>

当我求值时，我只得到Log In。

我想得到http://example.com/login

当我使用Python的lxml并计算该表达式时，它工作得很好。