How do I return $wpdb->get_results as json in WordPress?

[admin]

I created a custom table in wordpress called

custom_users

and I want to access the records via the API

<strong>functions.php</strong>

function get_wp_custom_users()
{
    global $wpdb;

    $custom_users = $wpdb->get_results("SELECT * FROM wp_custom_users");

    foreach ($custom_users as $custom_user)
    {
        echo $custom_user->first_name.' '.$custom_user->last_name;
    }
}

add_action('rest_api_init', function()
{
    register_rest_route( 'wpcustomusers/v1', '/users/', array(
        'methods' => 'GET',
        'callback' => 'get_wp_custom_users'
    ));
});

The API can be accessed via url

http://localhost/mywebsite/wp-json/wpcustomusers/v1/users

Do you know how can I return the results inside the

for loop

as JSON from the API request?

I hope to see all the fields returned.. Thanks for the help.