• převzato z https://developers.facebook.com/docs/authentication/signed_request/

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, '-_', '+/'));
}

• následně zavolat fci parse_signed_request($_REQUEST['signed_request'], $application_secret); a podívat se na klíč pole liked.