Standard Tag in Post @Username 

To Tag Friends on Facebook via Graph the massage needs to be in following format 

@[{user_id}:1:{name}]

If you want to tag an user with the ID 0123456789, having name Peter Metabaron, you have to include the following in your message tag:

@[{0123456789}:1:{Peter Metabaron}]

@[1234567890:1:Peter Metabaron]

achieved via the Graph API same for Fanpages 

To Tag a Fanpage or Virtual Fanpage like a Blogpost use the Fanpage UID or User UID

User Tag with Uid will not link or notify Text part is irrelevant

@[458025154231865:Social Media Tipps]

You can test that in the Facebook Graph explorer 

Tag of Chat objects in the chat window for example anim Emoticons Gifs

[[336842623036575]]

We can link blog graph words with the related uid see writing in Bold on facebook

Same format should work when upating Facebook via Twitter or Google Calendar Feeds -> Birthdays Greets. User can turn off Tagging in his Privacy settings.

Old Rest Api Examel

$uid = "0123456789"; //User's Facebook ID, set the value as "me" if publishing on logged in user's wall
$ufname = "Peter Metabaron"; // User's Full name on Facebook
$attachment = array(
'message'=> "Happy B-Day, @[".$uid.":1:".$ufname."]!",
);
try {
$post = $facebook->api('/'.$uid.'/feed', 'post', $attachment);
print_r($pp); //show the tagged post ID
} catch (FacebookApiException $e) {
//Error
exit;
}

New Graph version 

Facebook Photo Tagging Friends

Facebook Reference Photo Tag dev

PHP GD Libery Merge Photo imagecopymerge