In the context of Drupal, the difference will depend whether clean URLs are on or not.
With them off, $_SERVER['REQUEST_URI']
will have the full path of the page as called w/ /index.php
, while $_GET["q"]
will just have what is assigned to q
.
With them on, they will be nearly identical w/o other arguments, but $_GET["q"]
will be missing the leading /
. Take a look towards the end of the default .htaccess to see what is going on. They will also differ if additional arguments are passed into the page, eg when a pager is active.