Why does Simplepie return feed items in a wrong order?

The simplest fix for someone who is experiencing this issue is this:

function my_add_force_rss($feed,$url){
   $feed->force_feed(true);
   $feed->enable_order_by_date(false);
}
add_action('wp_feed_options', 'my_add_force_rss', 10,2);

Placing this in your functions.php will force any feed to be displayed in the order that it is received.

At the end of class-simplepie.php there is this line:

usort($items, array(get_class($urls[0]), 'sort_items'));

Which force-sorts feed elements, in case of multifeed.

The $feed->enable_order_by_date(false); is indeed very important and the key to things, BUT it gets disregarded (what I was experiencing) if I use multifeed. Source: in get_items() of class-simplepie.php there is this:

// If we want to order it by date, check if all items have a date, and then sort it
if ($this->order_by_date && empty($this->multifeed_objects))

I used multifeed accidentally for single URLs too. Remember it's a plugin where I didn't know if it was going to be single or multiple URLs so it seemed convenient to always pass an array to fetch_feed().

So it now looks something like this:

$rss_url = explode(',',$rss_url); // Accept multiple URLs

if(count($rss_url) == 1){ // New code
    $rss_url = $rss_url[0];
}

add_action('wp_feed_options', array($this, 'jig_add_force_rss'), 10,2);
$rss = fetch_feed($rss_url);
remove_action('wp_feed_options', array($this, 'jig_add_force_rss'), 10);

And the action is:

function jig_add_force_rss($feed,$url){
    $feed->force_feed(true);
    $feed->enable_order_by_date(false);
}

Your solutions were working becuase you passed the RSS url as a string and not as a one-element array. I compared them with how I was using the methods and this was the difference. Now it works with and without query string in the Picasa feed URL, always same as how it looks in the browser.

I have no idea what's happening with this feed but with this code

include_once( ABSPATH . WPINC . '/feed.php' );
$rss = fetch_feed( 'https://picasaweb.google.com/data/feed/base/user/110981376645512804522/albumid/5638371435917293985' );

foreach ($rss->get_items() as $item) {
    if ($enclosure = $item->get_enclosure()) {
        echo '<img src="'.$enclosure->get_link().'">';
    }
}

I get the same order that I get when opening that feed with my browser. If I leave the ?alt=rss&kind=photo&hl=en_US in the url, it gives the problems you described and the items are in the wrong order.

No idea why, but maybe you can work forward from here.

Edit: So in short, maybe the problem was the url of the feed?

Here's a hacky way to get the right feed:

$url = 'https://picasaweb.google.com/data/feed/base/user/110981376645512804522/albumid/5638371435917293985?alt=json';
// or https://picasaweb.google.com/data/feed/base/user/117541338986679044195/albumid/6129832223894216561?alt=json&kind=photo&hl=en_US&imgmax=800
$json = json_decode(file_get_contents($url));
$feed = (array)$json->feed;
foreach ($feed['entry'] as $entry) {
    $i = 0;
    foreach ($entry->content as $image) {
        if($i % 2 != 0) { echo '<img src="'.$image.'" />'; }
        $i++;
    }
}

So I switched to json feed and parse it kinda uglily. But it gets the job done :D the $entry->content gives both image type and source which is why there's that extra if($i % 2 != 0) to get only every other node.

Still, I have no clue why the RSS does that, but here's alternative way anyway.

edit: to add again you said you really couldn't parse the rss manually so I'll just say that you can add &prettyprint=true to the url to get a prettier feed if you'd like to read it.

edit number thousand: Alternative way!

include_once( ABSPATH . WPINC . '/feed.php' );
$rss = fetch_feed( 'https://picasaweb.google.com/data/feed/base/user/117541338986679044195/albumid/6129832223894216561?alt=rss&fields=item(media:group(media:content(@url)))' );

foreach ($rss->get_items() as $item) {
    if ($enclosure = $item->get_enclosure()) {
        //echo $enclosure->get_link().'<br>';
      echo '<img src="'.$enclosure->get_link().'">';
    }
}

by adding ?alt=rss&fields=item(media:group(media:content(@url))) I got the two feeds you have provided thus far to work correctly! I got all the funny information to try out from here.

I think you simply have to replace:

$feed->enable_order_by_date(false);

with

$rss->enable_order_by_date(false);

to disable the descending date ordering.

In the case of a feed error, the fetch_feed() function returns an instance of the \WP_Error class. Otherwise the output is an instance of the \SimplePie class and this class contains the enable_order_by_date() public method.

So you should use it like this:

// Fetch the feed:
$rss = fetch_feed( $feed_url );

// Make sure we don't have and WP_Error instance:
if ( ! is_wp_error( $rss ) )   
{
    // Disable the date ordering:
    $rss->enable_order_by_date( false );

    // ... etc ...
}

where we (implicitly) make sure we have an instance of the \SimplePie class.