GET method... screwups

I have a form that a user fills in, and I _have_ to pass it via GET in the url.... so if a user enters some extraordinarily funky characters, The fields after that field are screwed up... Now when i recieve fields normally, I do an ereg_replace (as people so kindly helped me with earlier) and that becomes all good and oracle friendly... but how can I prevent people from entering weird characters into the field so that the url doesnt get screwed up? Any help would be pachy keen Thanks Ed