display the name of user

Answer 1

The doc wiki has PHP examples for most common calls, including the one you're looking for.

http://wiki.developers.facebook.com/index.php/Users.getInfo#Example%5FRequests

Answer 2

All you need to do is add the following line to your code:

$userInfo = $facebook->api_client->users_getInfo($user_id, array('name'));

$name = $userInfo[0]['name'];

This is the same as calling the following FQL query (recommended if you are familiar with SQL as Facebook likes to change a lot of their API from time to time):

$facebook->api_client->fql_query("SELECT name FROM user WHERE uid='$user_id'");