POST /sites/$site/users/$user_id

curl \
 -H 'authorization: Bearer YOUR_API_TOKEN' \
 --data-urlencode 'roles=Array' \
 --data-urlencode 'first_name=Rocco' \
 --data-urlencode 'last_name=Tripaldi' \
 'https://public-api.wordpress.com/rest/v1/sites/30434183/user/23'

<?php
$options  = array (
  'http' => 
  array (
    'ignore_errors' => true,
    'method' => 'POST',
    'header' => 
    array (
      0 => 'authorization: Bearer YOUR_API_TOKEN',
      1 => 'Content-Type: application/x-www-form-urlencoded',
    ),
    'content' => 
     http_build_query(  array (
        'roles' => 
        array (
          0 => 
          array (
            0 => 'administrator',
          ),
        ),
        'first_name' => 'Rocco',
        'last_name' => 'Tripaldi',
      )),
  ),
);

$context  = stream_context_create( $options );
$response = file_get_contents(
	'https://public-api.wordpress.com/rest/v1/sites/30434183/user/23',
	false,
	$context
);
$response = json_decode( $response );
?>