Need Help with register_globals OFF

I'm trying to use the more secure "register_globals=Off" setting for some simple scripts that do authentication and use sessions. But I'm ready to rip all my hair out! Can someone recommend a good article online that clearly discusses what's different when you have register_globals=Off? I've spent hours looking through the PHP site but I'm just as confused as I was before I started. I thought this setting only affected global variables, but, there are all kinds of things not working properly, like the header() function. It just won't work at all with register_globals=Off, but works just fine when I switch it back on! Thanks.