Joseph Scott

A gradually growing list of people have run into a very odd problem using XML-RPC methods in WordPress, where the left angle bracket ( < ) gets stripped. There's been a fair bit of discussion about this on ticket #7771. The bottom line: the behavior of the PHP XML extension when built against newer versions of libxml2 changed, such that left angle brackets get stripped when parsing XML.

There’s been some back and forth between libxml2 folks (email list) and the PHP folks (bug 45996), with no real solution for those using the tainted versions of libxml2. So what are your options if you’ve got this problem? Here’s two:

• Stick with older, known to work versions of libxml2. It’s been reported by others that libxml2 <= 2.6.32 work. I've personally only tested up to 2.6.30, which has been working fine for me.
• Build the PHP XML module against the expat parser instead of libxml2

Both of these options require some server admin abilities and know how, making them unrealistic options for many WordPress users. Undoubtedly many hosting services will role out these newer versions of libxml2 as part of their regular updates. This will leave some WordPress users with sudden errors that weren’t there before.

As this was spurred by a change in behavior by libxml2, I think the ideal solution would be to provide a backwards compatible mode that would restore the old parsing mechanism (you know, the one that doesn’t strip angle brackets). Short of that happening perhaps the XML extension for PHP will need to grow to work correctly with the new way that libxml2 works. Either way, I’d like to see PHP XML parsing work correctly again.

If you aren’t having any of these problems right now I recommend NOT upgrading libxml2 on your system until this has been sorted out.

Today brings the official release of WordPress 2.7. I love what Michael Pick has been doing with the videos for the new WordPress releases:

There’s lots of information about the new UI and features in the announcement. Here’s what’s changed for XML-RPC when you compare the 2.6 branch to the new 2.7 code:

• New method: wp.getTags (ticket 7744)
• New comments API methods (ticket 7446):
  • wp.getComment
  • wp.getComments
  • wp.deleteComment
  • wp.editComment
  • wp.newComment
  • wp.getCommentStatusList
• Site ID fix for wp.getUsersBlogs (ticket 8255)
• New optional argument for wp.getPages - 4th parameter is now the maximum number of pages to return, which defaults to 10.
• metaWeblog.getPost now returns enclosure information (ticket 7772)
• Correct metaWeblog.getCategories to provide the actual comment description field (ticket 8494)

And the changes for AtomPub:

• Specify a status code of 401 when requiring authentication (ticket 7361)
• Work around for setups that do not basic HTTP basic authentication headers, like Apache when using PHP as a CGI (ticket 7361)

The new comments API methods are already getting some use. The next version of the WordPress iPhone App makes use of them for comment moderation. I also recently came across a WordPress comment moderation web app, which gives you way to manage WP comments outside of WP. Here’s a video demo.

If you have an interest in XML-RPC or AtomPub development in WordPress please join the wp-xmlrpc email list.

WordPress has an extensive set of actions and filters. All of the hooks allow you add/change/remove low level features via plugins and themes. Using this technique plugins can expose new XML-RPC methods for your WordPress blog.

In the xmlrpc.php file you’ll see a few lines that looks like:

        $this->methods = apply_filters('xmlrpc_methods', $this->methods);
        $this->IXR_Server($this->methods);

This allows plugins to filter the array of XML-RPC methods before the service starts. Adding a new function involves three parts: calling add_filter(), defining a function to manipulate the method list and a function that does the actual work behind the new method. I’ll go through a simple example, one step at a time.

add_filter( 'xmlrpc_methods', 'add_new_xmlrpc_methods' );

This tells WordPress to pass the xmlrpc_methods filter data to our function add_new_xmlrpc_methods.

function add_new_xmlrpc_methods( $methods ) {
    $methods['demo.multiplyTwoNumbers'] = 'multiply_two_numbers';
    return $methods;
}

This is our add_new_xmlrpc_methods function that does the filtering work. We’re adding a new key/value pair to the methods array. The key (demo.multiplyTwoNumbers) is the method name that is exposed via XML-RPC. The value (multiply_two_numbers) is the function that will do the work when that method is invoked.

function multiply_two_numbers( $args ) {
    $first  = (int) $args[0];
    $second = (int) $args[1];

    return $first * $second;
}

The final piece is the function that is called when our new XML-RPC method is invoked. The XML-RPC service will pass all of the arguments to your function as an array. After working with the provided parameters we return some value. The XML-RPC service will figure out what data type you are returning and encode it in appropriately way so that the client can understand it. This is means you don’t have to think about any of the XML-RPC level issues, just do the work and return the data.

Multiplying two numbers isn’t particularly interesting, so use your imagination. There are additional internal blog or WordPress information you could expose. For that matter you could run some of your data against third party APIs and then return the results.

To get you started here’s the entire plugin code:

<?php
/*
Plugin Name:    New XML-RPC Methods
Plugin URI:     http://josephscott.org/archives/2008/11/adding-xml-rpc-methods-to-wordpress
Description:    Demo how to add XML-RPC methods to WordPress.
Version:        0.1
Author:         Joseph Scott
Author URI:     http://josephscott.org/
 */

add_filter( 'xmlrpc_methods', 'add_new_xmlrpc_methods' );
function add_new_xmlrpc_methods( $methods ) {
    $methods['demo.multiplyTwoNumbers'] = 'multiply_two_numbers';
    return $methods;
}

function multiply_two_numbers( $args ) {
    $first  = (int) $args[0];
    $second = (int) $args[1];

    return $first * $second;
}

If you create a plugin to add new XML-RPC methods be sure to leave a comment letting us know what you came up with.