i am using the following code to publish to a Page\'s feed connection
feed
$attachment = array(\'message\' => \'xxxxxxxxxxxxxxxxx\',
The Post's id is stored in $result['id'].
$result['id']
It's the return value of the call to feed; this is also the case for /links, /photos, etc.
/links
/photos
Ref: http://developers.facebook.com/docs/reference/php/facebook-api/
Check the value in the $result variable, that should be the actual post_id value.