To make base64_encode encode a URL safe string compatible with .net HttpServerUtility.UrlTokenEncode function use this:

<?php
url_safe_base64_encode($string)
  {
    #First base64 encode
    $data = base64_encode($string);

    #Base64 strings can end in several = chars. These need to be translated into a number
    $no_of_eq = substr_count($data, "=");
    $data = str_replace("=", "", $data);
    $data = $data.$no_of_eq;

    #Then replace all non-url safe characters
    $data = str_replace(array('+','/'),array('-','_'),$data);
    return $data;
  }
?>