OK a number of small changes here.

OK a number of small changes here.

Firstly, I'm replacing all use of old-style global variables like
$HTTY_REFERER with their new-style equivalent $_SERVER["HTTP_REFERER"]
Also using $_POST instead $HTTP_POST_VARS  etc

Secondly, if gdversion == 0 (ie GD is not installed) then:

 - users are not even allowed to upload new images
 - graphs now just print a message instead of failing.

this allows Moodle to still be used even if GD is not present