add_action('rest_api_init', 'my_rest_validate_endpoint' );
function my_rest_validate_endpoint() {

    // Declare our namespace
    $namespace = 'myrest/v1';

    // Register the route
    // Example URL matching this route:
    // http://yourdomain/wp-json/myrest/v1/guides/tag=europe/price=29
    register_rest_route($namespace,
                        // Using regular expressions we can initially validate the input
                        '/guides/tag=(?P<tag>[a-zA-Z0-9-]+)/price=(?P<price>[0-9]+)',
                        // We can have multiple endpoints for one route
                        array(
                            array(
                                'methods'   => 'GET',
                                'callback'  => 'my_get_guides_handler'
                            )
                        ),
                        // We can register our schema callback
                        // 'schema' => 'my_get_guide_schema',
                
        );

    // You can register another route here the same way

}


// The callback handler for the endpoint
function my_get_guides_handler(WP_REST_Request $request) {

    // Get the parameters:
    $tag = $request->get_param('tag');
    $price = $request->get_param('price');

    // Do something with the parameters
    // for instance: get matching guides from the DB into an array - $results
    // ...
    
    // Prepare the response
    $message = "We've found " . count($results) . " guides ";
    $message .= "(searching for a tag: " . $tag . ", with a price tag: " . $price . ")";

    // The response gets automatically converted into a JSON format
    return new WP_REST_Response(
        array(
            'results' => $results,
            'message' => $message,
            'status' => 'OK'
        ),
        200 );

}