If you try to get a serialized array (php_serial) from the flicker api from a group (flickr.groups.pools.getPhotos), you may find your self a little bit lost, because you do not get the photos you want, only the amount of how many. You double check if everything is set to public, your account is ok and even try to upgrade it - but that's not it. You used the usefull API querybuilder from flicker and then it works in that case and you wonder why it doesn't in your application. The simple solution is, it doesn't work with the php_serial format. Skip this parameter and you get your set as XML response. The only thing you need to do is, use the XMLReader and iterate through the result set.

It took me quite a while to figure this out - like 1,5 hours - so eventually this post is usefull for someone ;)