php和xpath试图循环遍历特定元素的子元素

我有一个XML片段：

<ad:images gallery-url="http://myimage.com/adifferent/gallery.JPG" url="https://myxmlonly/images">
  <ad:image>
    <ad:representation size="S" url="http://myimage.com/adifferent/12345/S.JPG"/>
    <ad:representation size="XL" url="http://myimage.com/adifferent/12345/XL.JPG"/>
    <ad:representation size="ICON" url="http://myimage.com/adifferent/12345/ICON.JPG"/>
    <ad:representation size="L" url="http:/myimage.com/adifferent/12345/L.JPG"/>
    <ad:representation size="M" url="http://myimage.com/adifferent/12345/M.JPG"/>
  </ad:image>
  <ad:image>
    <ad:representation size="S" url="http://myimage.com/adifferent/67890/S.JPG"/>
    <ad:representation size="XL" url="http://myimage.com/adifferent/67890/XL.JPG"/>
    <ad:representation size="ICON" url="http://myimage.com/adifferent/67890/ICON.JPG"/>
    <ad:representation size="L" url="http:/myimage.com/adifferent/67890/L.JPG"/>
    <ad:representation size="M" url="http://myimage.com/adifferent/67890/M.JPG"/>
  </ad:image>
</ad:images>

我尝试了各种方法，但似乎我无法理解： 以下是产生以下内容的片段：

  //Get the images
  $images_array = array();

  //Path to images
  $images = $xml->xpath("//ad:images/*");

  //Starting index
  $ind = 0;

  //$image = $xml->xpath("//ad:image");
  //Iterate through images
  foreach($images as $node) {

    //Get pathes for all sizes
    $image = $node->xpath("//ad:image/*");

    //Iterate through size
    foreach($image as $imgSize) {
    //foreach($node->xpath("//ad:representation") as $imgSize) {

      //$size = $imgSize[0]['size'];
      $size = $imgSize[0]['size'];

      //Store size in array
      $images_array[$ind][(string)$size] = (string) $imgSize[0]['url'];
    }
    //Increse index
    $ind++;
  }

感谢阅读：）

我自己破解了它：

$ind = 0;
foreach($xml->xpath("//ad:image") as $image)
{
  foreach($image->xpath(".//ad:representation") as $imgSize)
  {
    $size = $imgSize['size'];
    $images_array[$ind][(string)$size] = (string)$imgSize['url'];
  }
  $ind++;
}

$ind = 0;
foreach($xml->xpath("//ad:image") as $image)
{
  foreach($image->xpath(".//ad:representation") as $imgSize)
  {
    $size = $imgSize['size'];
    $images_array[$ind][(string)$size] = (string)$imgSize['url'];
  }
  $ind++;
}