_activitystream_save() function is not compatible with PHP4 when using item["guid"]

When using modules that set with item["guid"] set (such as the StumbleUpon module when modified as per http://drupal.org/node/370383), _activitystream_save() calls the PHP built-in function md5() with two parameters. The second parameter (bool $raw_output) was not added until PHP 5.0.0 and results in the following error:

warning: Wrong parameter count for md5() in DRUPAL/sites/all/modules/contrib/activitystream/activitystream.module on line 448.

This call should either be updated, or activitystream.module should be updated with: php = 5.0