WordPress: Example of returning non-JSON results from the WP-API

multiformat-wp-api-response.php

<?php

// reference https://github.com/WP-API/WP-API/blob/develop/lib/infrastructure/class-wp-rest-server.php
// serve_request() function
add_filter( 'rest_pre_serve_request', 'multiformat_rest_pre_serve_request', 10, 4 );

function multiformat_rest_pre_serve_request( $served, $result, $request, $server ) {

	// assumes 'format' was passed into the intial API route
	// example: https://baconipsum.com/wp-json/baconipsum/test-response?format=text
	// the default JSON response will be handled automatically by WP-API

	switch ( $request['format'] ) {

		case 'text':
			header( 'Content-Type: text/plain; charset=' . get_option( 'blog_charset' ) );

			echo $result->data->my_text_data;
			$served = true; // tells the WP-API that we sent the response already
			break;

		case 'xml': // I guess if you really need to
			header( 'Content-Type: application/xml; charset=' . get_option( 'blog_charset' )  );

			$xmlDoc = new DOMDocument();
			$response = $xmlDoc->appendChild( $xmlDoc->createElement( 'Response' ) );
			$response->appendChild( $xmlDoc->createElement( 'My_Text_Data', $result->data->my_text_data ) );

			echo $xmlDoc->saveXML();
			$served = true;
			break;

	}

	return $served;

}