php - xpath问题中的上下文节点

have this code:

有这个代码:

$products   =   $feed->_xpath->query( "//cf:vehicle"  );

foreach( $products as $product )
{
    echo $product->nodeName . ': ' . $product->getAttribute('code') . '<br />';
    $imgs   =   $feed->_xpath->query( "//cf:image" , $product );
    echo '&nbsp;Imgs: ' . $imgs->length . '<br />';
}
$products   =   $fee