You can hide the post after you have posted it:
$fanpage_credit = array(
'access_token' => $page_token,
'is_hidden' => true
);
$fanpage_post_url = '/'.$post_id;
$postResult = $facebook->api($fanpage_post_url, 'post', $fanpage_credit );
if (isset($postResult["id"]))
$facebook->api("/$postResult[id]", 'post', array("timeline_visibility"=>"hidden"));