Why do some PHP installations have $_SERVER['SCRIPT_URI'] and others not?

Question

I run two Apache 2 servers. One has PHP5.2 and the other has PHP5.3. Is there a reason why on the 5.3 machine has $_SERVER['SCRIPT_URI']?

Where does this variable come from? It is clearly something that is coming through from the Apache environment and it is not documented in the PHP manual. It is however a handy shortcut over a combination of ['HTTPS'], ['SERVER_NAME'] and ['REQUEST_URI'].

I have tried looking through configuration files, searching SO and the web.

Answer 1

According to a post on WebHostingTalk it comes from mod_rewrite:

Add

RewriteEngine On

To the virtual host in your httpd.conf file that you want to turn this on for and then restart apache.