a way to encrypt URL?

Yes, you are right. As I said I was thinking about giving access to a single download or something, for a full member access a regular authentication scheme is the better way. Though... for mailing a login link to the user I would probablys use some kind of encrypted password in the link. A plain link like website.com/?user=me&password=secret might scare some users because it looks unsecure ... though it is not more unsecure than a link containing an md5 of the password