PHP class implementation for Outlook's bogus proprietary thread-index for message threading

outlook_thread_index.class.php

<?php   /*

  
  
  encode/decode Outlook thread index.
  GUID must be 32 bytes long
  
  forked from brettp/thread-index.php

*/


abstract class outlook_thread_index {

  static function build($guid, $timestamp=NULL){    // create one
    if(strlen($guid)!=32)throw new Exception("guid is not 32 bytes");

    $t = ($timestamp ? $timestamp : time() );
    $ft = ($t * 10000000) + 116444736000000000;
    // convert to hex and 0-pad to 8 bytes
    $ft_hex = base_convert($ft, 10, 16);
    $ft_hex = str_pad($ft_hex, 16, 0, STR_PAD_LEFT);
    // combine first 6 bytes of timestamp with hashed guid, convert to bin, then encode
    $thread_ascii = substr($ft_hex, 0, 12) . $guid;
    $thread_bin = hex2bin($thread_ascii);
    $thread_enc = base64_encode($thread_bin);
    return $thread_enc;
  }

  static function get_guid($thread_enc){   // return GUID
    $thread_bin = base64_decode($thread_enc);
    $thread_ascii = bin2hex($thread_bin);
    return
    $guid = substr($thread_ascii,12);
  }

}