What is the value in '$_SERVER['UNIQUE_ID'] used for?

Question

I can't find any mention of it in the documentation, but there seems to be an additional entry in the $_SERVER superglobal named UNIQUE_ID? What is this value used for?

It's fairly obvious that it's a unique ID, but of what? Of the current session, script, or server?

Answer 1

http://httpd.apache.org/docs/current/mod/mod_unique_id.html

Apache Module mod_unique_id

This module provides a magic token for each request which is guaranteed to be unique across "all" requests under very specific conditions. The unique identifier is even unique across multiple machines in a properly configured cluster of machines. The environment variable UNIQUE_ID is set to the identifier for each request. Unique identifiers are useful for various reasons which are beyond the scope of this document.