Adding XML-RPC Methods to WordPress

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:

[sourcecode language=”php”]
$this->methods = apply_filters(‘xmlrpc_methods’, $this->methods);
$this->IXR_Server($this->methods);
[/sourcecode]

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.

[sourcecode language=”php”]
add_filter( ‘xmlrpc_methods’, ‘add_new_xmlrpc_methods’ );
[/sourcecode]

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

[sourcecode language=”php”]
function add_new_xmlrpc_methods( $methods ) {
$methods[‘demo.multiplyTwoNumbers’] = ‘multiply_two_numbers’;
return $methods;
}
[/sourcecode]

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.

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

return $first * $second;
}
[/sourcecode]

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:

[sourcecode language=”php”]
<?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;
}
[/sourcecode]

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.