Determine If The User Likes (Fan Of) Your Facebook Page When Landing On Your Page Tab

It seems that a lot of developers and admins Facebook pages don’t know that they can detect whether the visiting user is already a fan of the page or not.

When the user visit your Page Tab, Facebook will send you the usual signed_request but with additional parameter called page (reference):

When a user navigates to the Facebook Page, they will see your Page Tab added in the next available tab position. Broadly, a Page Tab is loaded in exactly the same way as a Canvas Page. When a user selects your Page Tab, you will received the signed_request parameter with one additional parameter, page. This parameter contains a JSON object with an id (the page id of the current page), admin (if the user is a admin of the page), and liked (if the user has liked the page). As with a Canvas Page, you will not receive all the user information accessible to your app in the signed_request until the user authorizes your app.

This parameter is crucial for a successful product branding and user conversion. We can’t stress enough how important this feature is. How to make use of this feature is really up to you and your imagination and there are a lot of real world examples on Facebook like the YouTube Page and Linkin Park Page.

Make use of the liked parameter

Reading the signed_request and extracting the data we need is really simple:

<?php
if(empty($_REQUEST["signed_request"])) {
	// no signed request where found which means
	// 1- this page was not accessed through a Facebook page tab
	// 2- a redirection was made, so the request is lost
	echo "signed_request was not found!";
} else {
	$app_secret = "APP_SECRET";
	$data = parse_signed_request($_REQUEST["signed_request"], $app_secret);
	if (empty($data["page"]["liked"])) {
		echo "You are not a fan!";
	} else {
		echo "Welcome back fan!";
	}
}

function parse_signed_request($signed_request, $secret) {
	list($encoded_sig, $payload) = explode('.', $signed_request, 2); 

	// decode the data
	$sig = base64_url_decode($encoded_sig);
	$data = json_decode(base64_url_decode($payload), true);

	if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
		error_log('Unknown algorithm. Expected HMAC-SHA256');
		return null;
	}

	// check sig
	$expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
	if ($sig !== $expected_sig) {
		error_log('Bad Signed JSON signature!');
		return null;
	}

	return $data;
}

function base64_url_decode($input) {
	return base64_decode(strtr($input, '-_', '+/'));
}
?>

So with the code above you can detect if the user likes your page and if not encourage him to do so!

Notes

• You can’t get the user id from the signed_request of the landing user.
• Page Tabs can be tested locally, read our tutorial about Developing Facebook Application Locally for more information.