NB This answer uses the v2 API. New applications should write to the v3 API.
It is now possible to use the drive.files.list endpoint to list files belonging to a specific folder.
To do so, you can use the ?q=
query parameter as follows:
GET https://www.googleapis.com/drive/v2/files?q="'<FOLDER_ID>' in parents"
In PHP, that would be
$service->files->list(array('q' => "'<FOLDER_ID>' in parents"));