URL variables, and the $_GET[xxx] function...

I'm moving hosts for a freelance site I do... They have an events section, and it's sortable by date, name and category etc... To achieve this, I passed the search/sort variables via the URL... However, I'm testing the new server, and those variables are not being detected? I've put this code at teh top of the code on events page '$sort_by = $_GET[sort_by];' etc... And while that sorts out the problem, I'm curious as to why I need to do that, and if those changes will cause me probs elsewhere? Perhaps a rookie Q, but I guess we all learn something every day eh? Any thoughts? Tris... ********************************************************************* The information contained in this e-mail message is intended only for the personal and confidential use of the recipient(s) named above. If the reader of this message is not the intended recipient or an agent responsible for delivering it to the intended recipient, you are hereby notified that you have received this document in error and that any review, dissemination, distribution, or copying of this message is strictly prohibited. If you have received this communication in error, please notify us immediately by e-mail, and delete the original message. ***********************************************************************