I know how to make a post to a Facebook page via the API using PHP SDK, that is done like this:
$facebook->api(\'/xxxxxxxxxxx/feed\', \'post\', a
Check out this note regarding your question: http://developers.facebook.com/docs/api > Authorization > Page impersonation.
Long story short, you need the manage_pages extended permission.
manage_pages
And btw, have you checked this first ?