Gets filename to store data, associated to the token.
string The profile filename
protected function getFilename($token) {
// Uses 4 last characters, because first are mostly the same.
$folderA = substr($token, -2, 2);
$folderB = substr($token, -4, 2);
return $this->folder . '/' . $folderA . '/' . $folderB . '/' . $token;
}