base64-encoding in cookies?

Hi, A campus web server (not under my control) returns an authentication string in a cookie named AUTH. The cookie's value is an encrypted, base64 encoded string. Unfortunately, when I examine $_COOKIE['AUTH'], it is clear that all of the '+' characters have been replaced with a ' ' character in the base64 string. Why is this? Obviously, this corrupts the data and makes it impossible to base64-decode the string correctly. I believe this is a php issue and not, say, an apache issue because a perl program can correctly authenticate the same cookie based on perl's $ENV{'HTTP_COOKIE'}. i.e., the perl cookie contains the original '+'. Does anyone know how to make php (v5.1.5) do the right thing with base64 encoded cookies? Thanks Fletcher