PHP: $_GET contains variables not in $_REQUEST

When I do print_r($_GET) I see 4 variables, one of which is not seen in print_r($_REQUEST). On this page I have a relevant .htaccess rule:

RewriteRule ^de/something/(.+)/(.+)/$ /something/something_darkside.php?URLpage=rental&URLcategory=$1&URLsubcategory=$2 [NE]

The variables from this rule (URLpage, URLcategory, URLsubcategory) are all present as REQUEST variables. So I'm wondering where the extra GET variable comes from - could be through register globals?

EDIT: the additional parameter is 开发者_StackOverflowID with a numeric value (ex 2256) which is used to retrieve records from the DB.

EDIT2: ok the page now loads ok after turning register globals ON. I'm still struggling to understand the logic behind this.

Maybe someone mutated $_GET somewhere in the app.