According to http://www.php.net/manual/en/reserved.variables.php#reserved.variables.server, there's a variable called $_SERVER["QUERY_STRING"] (or just $QUERY_STRING if you have register_globals on) which contains just the query string the script was called with. So if you just want to check what's after the question mark, this would be more efficient.
Also, I think if you use $_SERVER["PHP_SELF"] you just get the name of the script (not the query string--the part after the "?") because one of its uses is making a link that always loads the same page, and if it contained the query string as well you could get undesired behavior.
