Retrieving information from a feed (we'll cover entries and items in the next section though they follow identical principals) uses a clearly defined API which is exactly the same regardless of whether the feed in question is RSS, RDF or Atom. The same goes for sub-versions of these standards and we've tested every single RSS and Atom version. While the underlying feed XML can differ substantially in terms of the tags and elements they present, they nonetheless are all trying to convey similar information and to reflect this all the differences and wrangling over alternative tags are handled internally by Zend_Feed_Reader presenting you with an identical interface for each. Ideally, you should not have to care whether a feed is RSS or Atom so long as you can extract the information you want.

Of course, we don't live in an ideal world so there may be times the API just does not cover what you're looking for. To assist you, Zend_Feed_Reader offers a plugin system which allows you to write Extensions to expand the core API and cover any additional data you are trying to extract from feeds. If writing another Extension is too much trouble, you can simply grab the underlying DOM or XPath objects and do it by hand in your application. Of course, we really do encourage writing an Extension simply to make it more portable and reusable, and useful Extensions may be proposed to the Framework for formal addition.

Here's a summary of the Core API for Feeds. You should note it comprises not only the basic RSS and Atom standards, but also accounts for a number of included Extensions bundled with Zend_Feed_Reader. The naming of these Extension sourced methods remain fairly generic - all Extension methods operate at the same level as the Core API though we do allow you to retrieve any specific Extension object separately if required.

Given the variety of feeds in the wild, some of these methods will undoubtedly return NULL indicating the relevant information couldn't be located. Where possible, Zend_Feed_Reader will fall back on alternative elements during its search. For example, searching an RSS feed for a modification date is more complicated than it looks. RSS 2.0 feeds should include a <lastBuildDate> tag and (or) a <pubDate> element. But what if it doesn't, maybe this is an RSS 1.0 feed? Perhaps it instead has an <atom:updated> element with identical information (Atom may be used to supplement RSS's syntax)? Failing that, we could simply look at the entries, pick the most recent, and use its <pubDate> element. Assuming it exists... Many feeds also use Dublin Core 1.0 or 1.1 <dc:date> elements for feeds and entries. Or we could find Atom lurking again.

The point is, Zend_Feed_Reader was designed to know this. When you ask for the modification date (or anything else), it will run off and search for all these alternatives until it either gives up and returns NULL, or finds an alternative that should have the right answer.

In addition to the above methods, all Feed objects implement methods for retrieving the DOM and XPath objects for the current feeds as described earlier. Feed objects also implement the SPL Iterator and Countable interfaces. The extended API is summarised below.