Zend_Feed_Writer is the sibling component to Zend_Feed_Reader responsible for generating feeds for output. It supports the Atom 1.0 specification (RFC 4287) and RSS 2.0 as specified by the RSS Advisory Board (RSS 2.0.11). It does not deviate from these standards. It does, however, offer a simple Extension system which allows for any extension and module for either of these two specifications to be implemented if they are not provided out of the box.

In many ways, Zend_Feed_Writer is the inverse of Zend_Feed_Reader. Where Zend_Feed_Reader focuses on providing an easy to use architecture fronted by getter methods, Zend_Feed_Writer is fronted by similarly named setters or mutators. This ensures the API won't pose a learning curve to anyone familiar with Zend_Feed_Reader.

As a result of this design, the rest may even be obvious. Behind the scenes, data set on any Zend_Feed_Writer Data Container object is translated at render time onto a DOMDocument object using the necessary feed elements. For each supported feed type there is both an Atom 1.0 and RSS 2.0 renderer. Using a DOMDocument class rather than a templating solution has numerous advantages, the most obvious being the ability to export the DOMDocument for additional processing and relying on PHP DOM for correct and valid rendering.

As with Zend_Feed_Reader, Zend_Feed_Writer is a standalone replacement for Zend_Feed's Builder architecture and is not compatible with those classes.

The architecture of Zend_Feed_Writer is very simple. It has two core sets of classes: data containers and renderers.

The containers include the Zend_Feed_Writer_Feed and Zend_Feed_Writer_Entry classes. The Entry classes can be attached to any Feed class. The sole purpose of these containers is to collect data about the feed to generate using a simple interface of setter methods. These methods perform some data validity testing. For example, it will validate any passed URIs, dates, etc. These checks are not tied to any of the feed standards definitions. The container objects also contain methods to allow for fast rendering and export of the final feed, and these can be reused at will.

In addition to the main data container classes, there are two additional Atom 2.0 specific classes. Zend_Feed_Writer_Source and Zend_Feed_Writer_Deleted. The former implements Atom 2.0 source elements which carry source feed metadata for a specific entry within an aggregate feed (i.e. the current feed is not the entry's original source). The latter implements the Atom Tombstones RFC allowing feeds to carry references to entries which have been deleted.

While there are two main data container types, there are four renderers - two matching container renderers per supported feed type. Each renderer accepts a container, and based on its content attempts to generate valid feed markup. If the renderer is unable to generate valid feed markup, perhaps due to the container missing an obligatory data point, it will report this by throwing an Exception. While it is possible to ignore Exceptions, this removes the default safeguard of ensuring you have sufficient data set to render a wholly valid feed.

To explain this more clearly, you may construct a set of data containers for a feed where there is a Feed container, into which has been added some Entry containers and a Deleted container. This forms a data hierarchy resembling a normal feed. When rendering is performed, this hierarchy has its pieces passed to relevant renderers and the partial feeds (all DOMDocuments) are then pieced together to create a complete feed. In the case of Source or Deleted (Tomestone) containers, these are rendered only for Atom 2.0 and ignored for RSS.

Due to the system being divided between data containers and renderers, it can make Extensions somewhat interesting. A typical Extension offering namespaced feed and entry level elements, must itself reflect the exact same architecture, i.e. offer feed and entry level data containers, and matching renderers. There is, fortunately, no complex integration work required since all Extension classes are simply registered and automatically used by the core classes. We'll meet Extensions in more detail at the end of this section.

Using Zend_Feed_Writer is as simple as setting data and triggering the renderer. Here is an example to generate a minimal Atom 1.0 feed. As this demonstrates, each feed or entry uses a separate data container.

The output rendered should be as follows:

This is a perfectly valid Atom 1.0 example. It should be noted that omitting an obligatory point of data, such as a title, will trigger an Exception when rendering as Atom 1.0. This will differ for RSS 2.0 since a title may be omitted so long as a description is present. This gives rise to Exceptions that differ between the two standards depending on the renderer in use. By design, Zend_Feed_Writer will not render an invalid feed for either standard unless the end-user deliberately elects to ignore all Exceptions. This built in safeguard was added to ensure users without in-depth knowledge of the relevant specifications have a bit less to worry about.

Before you can render a feed, you must first setup the data necessary for the feed being rendered. This utilises a simple setter style API which doubles as an initial method for validating the data being set. By design, the API closely matches that for Zend_Feed_Reader to avoid undue confusion and uncertainty.

Note: Users have commented that the lack of a simple array based notation for input data gives rise to lengthy tracts of code. This will be addressed in a future release.

Zend_Feed_Writer offers this API via its data container classes Zend_Feed_Writer_Feed and Zend_Feed_Writer_Entry (not to mention the Atom 2.0 specific and Extension classes). These classes merely store all feed data in a type-agnostic manner, meaning you may reuse any data container with any renderer without requiring additional work. Both classes are also amenable to Extensions, meaning that an Extension may define its own container classes which are registered to the base container classes as extensions, and are checked when any method call triggers the base container's __call() method.

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_Writer. 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.

The Feed Level API for data is contained in Zend_Feed_Writer_Feed. In addition to the API detailed below, the class also implements the Countable and Iterator interfaces.

Feed Level API Methods

setId()	Set a unique ID associated with this feed. For Atom 1.0 this is an atom:id element, whereas for RSS 2.0 it is added as a guid element. These are optional so long as a link is added, i.e. the link is set as the ID.
setTitle()	Set the title of the feed.
setDescription()	Set the text description of the feed.
setLink()	Set a URI to the HTML website containing the same or similar information as this feed (i.e. if the feed is from a blog, it should provide the blog's URI where the HTML version of the entries can be read).
setFeedLinks()	Add a link to an XML feed, whether the feed being generated or an alternate URI pointing to the same feed but in a different format. At a minimum, it is recommended to include a link to the feed being generated so it has an identifiable final URI allowing a client to track its location changes without necessitating constant redirects. The parameter is an array of arrays, where each sub-array contains the keys "type" and "uri". The type should be one of "atom", "rss", or "rdf".
addAuthors()	Sets the data for authors. The parameter is an array of arrays where each sub-array may contain the keys "name", "email" and "uri". The "uri" value is only applicable for Atom feeds since RSS contains no facility to show it. For RSS 2.0, rendering will create two elements - an author element containing the email reference with the name in brackets, and a Dublin Core creator element only containing the name.
addAuthor()	Sets the data for a single author following the same array format as described above for a single sub-array.
setDateCreated()	Sets the date on which this feed was created. Generally only applicable to Atom where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or a Zend_Date object.
setDateModified()	Sets the date on which this feed was last modified. The expected parameter may be a UNIX timestamp or a Zend_Date object.
setLastBuildDate()	Sets the date on which this feed was last build. The expected parameter may be a UNIX timestamp or a Zend_Date object. This will only be rendered for RSS 2.0 feeds and is automatically rendered as the current date by default when not explicity set.
setLanguage()	Sets the language of the feed. This will be omitted unless set.
setGenerator()	Allows the setting of a generator. The parameter should be an array containing the keys "name", "version" and "uri". If omitted a default generator will be added referencing Zend_Feed_Writer, the current Zend Framework version and the Framework's URI.
setCopyright()	Sets a copyright notice associated with the feed.
addHubs()	Accepts an array of Pubsubhubbub Hub Endpoints to be rendered in the feed as Atom links so that PuSH Subscribers may subscribe to your feed. Note that you must implement a Pubsubhubbub Publisher in order for real-time updates to be enabled. A Publisher may be implemented using Zend_Feed_Pubsubhubbub_Publisher. The method addHub() allows adding a single hub at a time.
addCategories()	Accepts an array of categories for rendering, where each element is itself an array whose possible keys include "term", "label" and "scheme". The "term" is a typically a category name suitable for inclusion in a URI. The "label" may be a human readable category name supporting special characters (it is HTML encoded during rendering) and is a required key. The "scheme" (called the domain in RSS) is optional but must be a valid URI. The method addCategory() allows adding a single category at a time.
setImage()	Accepts an array of image metadata for an RSS image or Atom logo. Atom 1.0 only requires a URI. RSS 2.0 requires a URI, HTML link, and an image title. RSS 2.0 optionally may send a width, height and image description. The array parameter may contain these using the keys: uri, link, title, description, height and width. The RSS 2.0 HTML link should point to the feed source's HTML page.
createEntry()	Returns a new instance of Zend_Feed_Writer_Entry. This is the Entry level data container. New entries are not automatically assigned to the current feed, so you must explicitly call addEntry() to add the entry for rendering.
addEntry()	Adds an instance of Zend_Feed_Writer_Entry to the current feed container for rendering.
createTombstone()	Returns a new instance of Zend_Feed_Writer_Deleted. This is the Atom 2.0 Tombstone level data container. New entries are not automatically assigned to the current feed, so you must explicitly call addTombstone() to add the deleted entry for rendering.
addTombstone()	Adds an instance of Zend_Feed_Writer_Deleted to the current feed container for rendering.
removeEntry()	Accepts a parameter indicating an array index of the entry to remove from the feed.
export()	Exports the entire data hierarchy to an XML feed. The method has two parameters. The first is the feed type, one of "atom" or "rss". The second is an optional boolean to set whether Exceptions are thrown. The default is TRUE.

Note: In addition to these setters, there are also matching getters to retrieve data from the Entry data container. For example, setImage() is matched with a getImage() method.

Here's a summary of the Core API for Entries and Items. 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_Writer. 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.

The Entry Level API for data is contained in Zend_Feed_Writer_Entry.

Entry Level API Methods

setId()	Set a unique ID associated with this entry. For Atom 1.0 this is an atom:id element, whereas for RSS 2.0 it is added as a guid element. These are optional so long as a link is added, i.e. the link is set as the ID.
setTitle()	Set the title of the entry.
setDescription()	Set the text description of the entry.
setContent()	Set the content of the entry.
setLink()	Set a URI to the HTML website containing the same or similar information as this entry (i.e. if the feed is from a blog, it should provide the blog article's URI where the HTML version of the entry can be read).
setFeedLinks()	Add a link to an XML feed, whether the feed being generated or an alternate URI pointing to the same feed but in a different format. At a minimum, it is recommended to include a link to the feed being generated so it has an identifiable final URI allowing a client to track its location changes without necessitating constant redirects. The parameter is an array of arrays, where each sub-array contains the keys "type" and "uri". The type should be one of "atom", "rss", or "rdf". If a type is omitted, it defaults to the type used when rendering the feed.
addAuthors()	Sets the data for authors. The parameter is an array of arrays where each sub-array may contain the keys "name", "email" and "uri". The "uri" value is only applicable for Atom feeds since RSS contains no facility to show it. For RSS 2.0, rendering will create two elements - an author element containing the email reference with the name in brackets, and a Dublin Core creator element only containing the name.
addAuthor()	Sets the data for a single author following the same format as described above for a single sub-array.
setDateCreated()	Sets the date on which this feed was created. Generally only applicable to Atom where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or a Zend_Date object. If omitted, the date used will be the current date and time.
setDateModified()	Sets the date on which this feed was last modified. The expected parameter may be a UNIX timestamp or a Zend_Date object. If omitted, the date used will be the current date and time.
setCopyright()	Sets a copyright notice associated with the feed.
addCategories()	Accepts an array of categories for rendering, where each element is itself an array whose possible keys include "term", "label" and "scheme". The "term" is a typically a category name suitable for inclusion in a URI. The "label" may be a human readable category name supporting special characters (it is encoded during rendering) and is a required key. The "scheme" (called the domain in RSS) is optional but must be a valid URI. The method addCategory() allows adding a single category at a time.
setCommentCount()	Sets the number of comments associated with this entry. Rendering differs between RSS and Atom 2.0 depending on the element or attribute needed.
setCommentLink()	Seta a link to a HTML page containing comments associated with this entry.
setCommentFeedLink()	Sets a link to a XML feed containing comments associated with this entry. The parameter is an array containing the keys "uri" and "type", where the type is one of "rdf", "rss" or "atom".
setCommentFeedLinks()	Same as setCommentFeedLink() except it accepts an array of arrays, where each subarray contains the expected parameters of setCommentFeedLink().
setEncoding()	Sets the encoding of entry text. This will default to UTF-8 which is the preferred encoding.

Note: In addition to these setters, there are also matching getters to retrieve data from the Entry data container.