Improvement on "gutzmer at usa dot net", "biziclop at vipmail dot hu", and "ivanm at duck dot com".

<?php
function base64url_encode($data) {
  return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
}

function base64url_decode($data) {
  return base64_decode(strtr($data, '-_', '+/'));
}
?>

None of the padding for strings longer than 4 characters worked. Padding the decode function with = is unnecessary, and has been since at least PHP 5.4 (as far back as I checked before posting) so I removed it.