Check if user has liked your page via Facebook API

If you wish to display some content that only those facebook users can see who liked your page, then you need to write this little piece of code to verify:

$signed_request = $_REQUEST["signed_request"];
list($encoded_sig, $payload) = explode('.', $signed_request, 2);
$data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);

if (empty($data["page"]["liked"])) {
    //do if user has liked your page
}else{
  //do if user has not liked your page
}

This technique is good, if you want to have more likes on your facebook pages.