rest-internal-request.php

<?php

/**
 * Make an internal REST request
 *
 * @global WP_REST_Server $wp_rest_server ResponseHandler instance (usually WP_REST_Server).
 * @param $request_or_method WP_REST_Request|string A WP_REST_Request object or a request method
 * @param $path (optional) if the path is not specific in the rest request, specify it here
 * @param $data (optional) option data for the request.
 * @return WP_Error|mixed
 */
function rest_internal_request( $request_or_method, $path = null, $data = array() ) {

	/** @var WP_REST_Server $wp_rest_server */
	global $wp_rest_server;

	if ( empty( $wp_rest_server ) ) {
		/**
		 * Filter the REST Server Class.
		 *
		 * This filter allows you to adjust the server class used by the API, using a
		 * different class to handle requests.
		 *
		 * @since 4.4.0
		 *
		 * @param string $class_name The name of the server class. Default 'WP_REST_Server'.
		 */
		$wp_rest_server_class = apply_filters( 'wp_rest_server_class', 'WP_REST_Server' );
		$wp_rest_server = new $wp_rest_server_class;

		/**
		 * Fires when preparing to serve an API request.
		 *
		 * Endpoint objects should be created and register their hooks on this action rather
		 * than another action to ensure they're only loaded when needed.
		 *
		 * @since 4.4.0
		 *
		 * @param WP_REST_Server $wp_rest_server Server object.
		 */
		do_action( 'rest_api_init', $wp_rest_server );
	}

	if ( is_a( $request_or_method, 'WP_REST_Request' ) ) {
		$request = $request_or_method;
	} else {
		$request = new WP_REST_Request( $request_or_method, $path );

		foreach ( $data as $k => $v ) {
			$request->set_param( $k, $v );
		}
	}

	$result = $wp_rest_server->dispatch( $request );

	if ( $result->is_error() ) {
		return $result->as_error();
	}

	$result = $wp_rest_server->response_to_data( $result, ! empty( $data['_embed'] ) );

	return $result;
}

Using the REST API internally within PHP can be useful for a number of cases. For example, pre-loading REST API data in the body of a document so there's no need to make initial API calls:

wp_localize_script( 'myScript', 'Tags', array(
    'tags' => rest_internal_request( 'GET', '/wp/v2/terms/tags', array( 'per_page' => 20 ) ),
) );