PHP: How to detect/parse all TAGS in rss/feed XML?

I'm testing rss feed reader, from a sample. It uses:

$xmlDoc = new DOMDocument();
$xmlDoc->load($url);

$items = $xmlDoc->getElementsByTagName('item');
for ($i=0; $i < $items->length; $i++) {
    $item_title = $items->item($i)->getElementsByTagName('title')->item(0)->childNodes->item(0)->nodeValue;
    $item_link  = $items->item($i)->getElementsByTagName('link')->item(0)->childNodes->item(0)->nodeValue;
    $item_desc  = $items->item($i)->getElementsByTagName('description')->item(0)->childNodes->item(0)->nodeValue;
}

When i check the XML, it has another Tags like:

• Date
• Image Link

How can i call these all other tags? Because i can't call myself.
For example, for the 'date', i can't use getElementsByTagName('date').
It showing error: Fatal error: Call to a member function item() on a non-object

• So, is there fixed names for tags?
• If so, what are these?
• (or) How can i know/ parse/ detact/ extract all available tags inside XML?