Newbie Questionabout variables

I hope this posting is not a duplicate. I originally posted it yesterday, but it doesn't show on the newsgroup for me. Apparently you can set a variable as part of a URL statement. For example: www.company.com/index.php?admin=password. Where "password" might be a simple variable to test as a rudimentary authorization password. I can't find anything like this on the php website or manual. Can someone point me in the right direction? Thanks....