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['signed_request'] = 'Unknown algorithm. Expected HMAC-SHA256';
    return null;
  }

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

  return $data;
}

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

$signed_request_data = parse_signed_request($_REQUEST['signed_request'],$fb_app_secret);

if($signed_request_data['page']['liked']) {
  print "Content for Useres who have liked your page...";
}