Is there a way to retrieve the list of Facebook users that have clicked the like button on an external website?
FQL Should do the trick.
$facebook->api_client->fql_query('SELECT user_id FROM like WHERE object_id="OBJECTID"');
Here is the link.
Some general info about FQL.
FQL is initiated using the JavaScript SDK, in this way.
If you can post some sample code-I can try and give some more specific help.
A note should be made-once you've got the user ID's, you can just query the names and get them in a list.
EDIT: To get the URL of an object you should query this table using fql.
This cannot be done. Facebook does not allow you to query for specific user ID's of people who have liked a certain page. This is because of privacy concerns.
See the disclaimer on this page https://developers.facebook.com/docs/reference/fql/like/