test-rest-endpoint.php

<?php

const REST_NAMESPACE = 'test/v1';

add_action( 'rest_api_init', function () {
	register_rest_route( REST_NAMESPACE, '/test', [
		'methods' => 'GET',
		'callback' => 'handle_rest_test',
		'permission_callback' => '__return_true',
	] );
} );

/**
 * Handle REST API test endpoint request.
 *
 * @return \WP_REST_Response
 */
function handle_rest_test() : \WP_REST_Response {
	return new \WP_REST_Response( [
		'message' => 'Hello from REST API',
	] );
}