Custom APIs are essential for adding web application-specific behaviors, which go beyond the generic blogging functionality. We need the implementation for both the server and client to create a custom API. Here, we will build an API function that outputs the list of topics of a specified forum in the forum application. Here, we will use a separate plugin for API creation as an API is usually a separate component from the application. Let's get started by creating another plugin folder called wpwa-xml-rpc-api with the main file called class-wpwa-xml-rpc-api.php. Since this is a plugin with a basic set of functions, we don't use the plugin coding structure used in the forum plugin.

Let's look at the initial code to build the API server:

    class WPWAF_XML_RPC_API { 
      public function __construct() { 
        add_filter('xmlrpc_methods', array($this, 'xml_rpc_api...