Best way to parse RSS/Atom feeds with PHP

I've always used the SimpleXML functions built in to PHP to parse XML documents. It's one of the few generic parsers out there that has an intuitive structure to it, which makes it extremely easy to build a meaningful class for something specific like an RSS feed. Additionally, it will detect XML warnings and errors, and upon finding any you could simply run the source through something like HTML Tidy (as ceejayoz mentioned) to clean it up and attempt it again.

Consider this very rough, simple class using SimpleXML:

class BlogPost
{
    var $date;
    var $ts;
    var $link;

    var $title;
    var $text;
}

class BlogFeed
{
    var $posts = array();

    function __construct($file_or_url)
    {
        $file_or_url = $this->resolveFile($file_or_url);
        if (!($x = simplexml_load_file($file_or_url)))
            return;

        foreach ($x->channel->item as $item)
        {
            $post = new BlogPost();
            $post->date  = (string) $item->pubDate;
            $post->ts    = strtotime($item->pubDate);
            $post->link  = (string) $item->link;
            $post->title = (string) $item->title;
            $post->text  = (string) $item->description;

            // Create summary as a shortened body and remove images, 
            // extraneous line breaks, etc.
            $post->summary = $this->summarizeText($post->text);

            $this->posts[] = $post;
        }
    }

    private function resolveFile($file_or_url) {
        if (!preg_match('|^https?:|', $file_or_url))
            $feed_uri = $_SERVER['DOCUMENT_ROOT'] .'/shared/xml/'. $file_or_url;
        else
            $feed_uri = $file_or_url;

        return $feed_uri;
    }

    private function summarizeText($summary) {
        $summary = strip_tags($summary);

        // Truncate summary line to 100 characters
        $max_len = 100;
        if (strlen($summary) > $max_len)
            $summary = substr($summary, 0, $max_len) . '...';

        return $summary;
    }
}

With 4 lines, I import a rss to an array.

$feed = implode(file('http://yourdomains.com/feed.rss'));
$xml = simplexml_load_string($feed);
$json = json_encode($xml);
$array = json_decode($json,TRUE);

For a more complex solution

$feed = new DOMDocument();
 $feed->load('file.rss');
 $json = array();
 $json['title'] = $feed->getElementsByTagName('channel')->item(0)->getElementsByTagName('title')->item(0)->firstChild->nodeValue;
 $json['description'] = $feed->getElementsByTagName('channel')->item(0)->getElementsByTagName('description')->item(0)->firstChild->nodeValue;
 $json['link'] = $feed->getElementsByTagName('channel')->item(0)->getElementsByTagName('link')->item(0)->firstChild->nodeValue;
 $items = $feed->getElementsByTagName('channel')->item(0)->getElementsByTagName('item');

 $json['item'] = array();
 $i = 0;

 foreach($items as $key => $item) {
 $title = $item->getElementsByTagName('title')->item(0)->firstChild->nodeValue;
 $description = $item->getElementsByTagName('description')->item(0)->firstChild->nodeValue;
 $pubDate = $item->getElementsByTagName('pubDate')->item(0)->firstChild->nodeValue;
 $guid = $item->getElementsByTagName('guid')->item(0)->firstChild->nodeValue;

 $json['item'][$key]['title'] = $title;
 $json['item'][$key]['description'] = $description;
 $json['item'][$key]['pubdate'] = $pubDate;
 $json['item'][$key]['guid'] = $guid; 
 }

echo json_encode($json);