Create custom REST route

hello_world.php

<?php
add_action( 'rest_api_init', 'my_register_route' );

function my_register_route() {
    register_rest_route( 'my-route', 'my-phrase', array(
                    'methods' => 'GET',
                    'callback' => 'custom_phrase',
                )
            );
}

function custom_phrase() {
    return rest_ensure_response( 'Hello World! This is my first REST API' );
}
?>