[API] Can I make API requests from PHP within a plugin? (ok)

https://developer.wordpress.org/rest-api/frequently-asked-questions/

C:\xampp\htdocs\api\wp-content\themes\twentyseventeen\header.php

<?php  
	$request = new WP_REST_Request( 'GET', '/wp/v2/posts' );
	// Set one or more request query parameters
	$request->set_param( 'per_page', 20 );
	$response = rest_do_request( $request );
	echo '<pre>';
		print_r($response);
	echo '</pre>';
?>